Liquid nitrogen ice cream

For the potluck on the last day of school, I made ice cream using liquid nitrogen.  Basic recipe: milk, cream, sugar.  For strawberry ice cream I added vanilla plus strawberry puree, and for ginger ice cream I added ginger syrup and ginger bits.  The LN2 cools it down real fast.  It was a big hit.

Planets, planets, planets

Last Friday Vera visited primaria.  The kids who stay for afternoon care had been learning about planets and stars there, and on my last visit I could see they were really excited about it.  For example, at snack time I was showing them pictures of Darth Crater on my phone:

(This is Mimas, a moon of Saturn; the crater's name is not actually Darth Crater but it's a good joke for kids.)

So, although generally we prefer hands-on activities rather than powerpoint, I thought this was one case where a slide show kind of presentation was appropriate.  So Vera gave a highly interactive tour of the solar system slide show, to two groups of 10-11 each.  The kids were super excited and tired her out!

One way to extend this in the future would be to have kids draw pictures of what they imagine other planets to be like, before and/or after the slide show, and see what their preconceptions are as well as how their conceptions changed.

A better way for kids to build circuits

I just learned about this from a fellow astronomer and part-time kid science instructor: squishy circuits.  It's worth watching the 4-minute video.  Next year, this is definitely the technology I'll use to get little hands to build circuits without frustration!

Siphon Tank

The school year is coming to an end and I never got time to do an activity based on this idea, so here is a quick video.

Focus, kids!

The Primaria kids are learning about solar ovens; they fried an egg
yesterday and they baked cookies today.  I brought in a neat piece of
equipment to complement that: a parabolic mirror about 24" in
diameter.  If you point it at the Sun and put anything flammable at
the focus, it will burst into flame in about one second.

I also brought a larger flat mirror and started by having the kids try
to figure out how to set a piece of paper on fire.  (I knew they
wouldn't actually be able to do it without guidance, so there were no
safety issues at this stage.)  After watching what they did, we
discussed some very basic physics:

--you can't have too many people crowding around the mirror, just as you
  wouldn't put the mirror under a tree

--you want to point the mirror at the Sun

So I pointed the larger flat mirror at the Sun and had them try again.
When they couldn't, I pointed out how the "small" mirror was curved
and explained why that was important: light hitting the edge of the
mirror will be bounced toward a point above the center, and light
hitting the center will also be be bounced toward a point above the
center.  All the light meets at one point! 

With that in mind, I had the kids form a line and I helped them ignite
paper one by one for safety reasons (most of them were also too scared
to try by themselves).  I showed how putting the paper directly on the
mirror was no different than just putting the paper in regular
sunlight. As we lifted the paper above the surface of the mirror we
could see the light start to focus down on a smaller area of the
paper.  At just the right distance from the mirror, all the light is
focused on such a small area of the paper that it starts to smoke!

This one-by-one part of the activity took most of the time.  There was
little or no time for many of the add-ons I had imagined, such as
connecting with the idea of a magnifying glass, discussing how to
store solar energy so you can use it at night, relating to the
temperatures of different planets (concentrating the light mimics the
effect of being closer to the Sun), and having them draw how it works.
With some of the groups, I had brief discussions of some of these
ideas, but we definitely could have used more than the allotted 20
minutes.  I especially wish we could have done the drawings...it would
be really interesting to see what the kids would come up with.

I wore pretty dark sunglasses for this activity.  At one point I took
my sunglasses off to lend them to a child, and I was just about
blinded...I did have a blind spot for about 5 minutes afterward.  So I
would recommend bringing sunglasses for the kids too.

You don't necessarily need a mirror as large as the one I was able to borrow.  I've seen survival shows where people used parabolic mirrors as small as the ones in flashlights to start fires.  It just requires more patience, and more fine motor skills to hit that small focus.

If you want to add a little more physics, note that the Sun provides over one kilowatt per square meter (the "solar constant" is 1.361 kW/m2, but the atmosphere takes a bit of that).  Kids in the upper elementary grades should be able to figure out how many watts their mirror collects and compare that with the power of their microwave oven, the energy consumption of a typical house, etc.

Liquid nitrogen

Last Friday (May 25) Vera visited Primaria and brought liquid nitrogen, which is as cold as Uranus or Neptune.  Pluto might warm up close to liquid nitrogen temperatures during its summer, but in its fall it gets a lot colder and nitrogen is thought to freeze out of its atmosphere onto its surface.  Fun stuff you can do with LN2 includes: (1) freezing a banana hard enough to use it as a hammer and pound a nail into a piece of wood; (2) make a balloon completely flaccid as the air inside cools...blow on it to warm it up and it pops back into its normal state; (3) freeze a racquetball and watch it shatter when thrown on the ground; (4) make ice cream instantly (Vera didn't do that one); (5) freeze a flower and see how it shatters when frozen; (6) freeze anything the kids suggest.  Vera also related it to the infrared camera I showed the kids earlier this year.  What color would LN2 appear on an infrared camera?

This is also a great demo for elementary (and older) kids, although we haven't done it there yet.   Keep in mind, it is a demo, not a hands-on activity, although you can definitely involve the kids in thinking of what to freeze next and making predictions for what will happen,

Planets and their orbits

Today I brought the coin funnel to Primaria and we learned how orbits work.  One four-year-old came up with Kepler's 3rd Law all by himself!  With 6-8 kids in each group I organized it by having each child raise a quiet hand, tell me something they know about planets or the Sun, and then I gave that child a marble to roll in the well.  That gave me a chance to comment on what we could learn from each marble.  Depending on how it was thrown, the lesson could have been about escape velocity, about planets not falling straight into the Sun, about all the planets going around in the same direction, etc. Most of the time we pretended the Sun was at the center but if you want to make it more exciting you can pretend it's a black hole. 

Then  we shifted gears and built a scale model of the solar system: I showed a basketball representing the Sun, and I laid out various objects (a tomato, a grape, a small candy, etc) for them to choose which one they thought was the right size to represent Jupiter, the Earth, etc.   The correct answers are truly amazing: learn more at this well-written site. We didn't have time to go outside and put the correct distances between the planets, but I did describe the highlights of that aspect: Earth would be roughly in the office if we were with the Sun in the P1 room.

Aside on teaching and learning: one aspect of orbits is that all objects in a given orbit go at the same speed, regardless of their mass (technically, as long as the mass is much smaller than the mass of the thing being orbited).  To reinforce that, I dropped two marbles of very different mass and asked which one would hit the ground first.  Of course, many had the misconception that the heavy one would hit first.  I remark on it now because I had done the same demo with the same kids eight weeks ago in a slightly different context, so it's clear that they forgot.  These misconceptions are persistent!

More static electricity

Today I visited Primaria and did an abbreviated, simplified version of the static electricity work I did with the elementary.  The bare essentials are to demonstrate attraction (hair to rubbed balloon) and then show that, counterintuitively, two rubbed balloons do not attract doubly but actually repel.  This leads us to conclude that there must be two kinds of charge and that like repels like.  I also did the deflection-of-water demo because it's too cool to miss, and it demonstrates that all things contain two kinds of charge even if on balance they are uncharged.

I also emphasized that electricity is stronger than gravity (a wimpy balloon overcomes the entire Earth's pull of gravity on the hair) so they could be hurt if they play with it without a grownup around.

One thing I did differently is that I immediately made a connection to magnetism.  They had played with magnets with their regular teacher, so I thought they might be able to make the connection themselves.  But when I asked what other thing (other than static electricity, whose dual nature we had just established) sometimes attracts and sometimes repels, they drew a blank.  I reminded them of magnets and noted the similarity between +/- and north/south, saying that there is a deep connection but they might have to be older to understand it.

That led nicely to the last 5-10 minutes, in which they played with magnets and static electricity, doing their own experiments.  There are a lot of fun things they can build, like anti-gravity devices (opposing ring magnets threaded onto a vertical pencil, which keeps them from sliding sideways), magnet bombs (stacks of opposing magnets forced together by hand, then suddenly released), and remote-control devices (a magnet on top of a tray manipulated by an unseen magnet below the tray).  They just need a little bit of hinting to start exploring the possibilities.

Addendum: the effectiveness of the static electricity demos varies quite a bit from day to day depending on the humidity.  If you have any flexibility, save it for a dry day.

Dinosaur layer cake

Some of the boys in Primaria are really into dinosaurs and have been
asking for a dinosaur-related experiment.  By talking to them on
previous visits, I got a sense of what would be useful.  They knew
that dinosaurs did not live at the same time as cavemen, but they
didn't know how we know that.  Understanding this brings together a
lot of key ideas in geology and in scientific reasoning, so I thought
it would make a great activity.  But it turned out to be more of a
demo than a small-group activity, so it fit the schedule well on a day
when there was less time for science due to the Easter egg hunt.

I brought a large, clear plastic box and set it on a table in the
outdoor area.  As part of the setup I also filled some buckets with
different materials in the yard: sand, wood chips, and black dirt from
the planter boxes.  I started, as usual, by asking them what they know
about the topic, and I tried to steer the resulting conversation
toward how they know what they know. (Aside: this is one of the few
times I had a conversation with the entire class of 20+ kids at once,
and it was surprisingly not chaotic.  It really helped to have them
seated before the start, with everyone able to see because I was on a
platform.)  One boy was able to give an answer like "men hadn't
evolved yet" but no one know how we know that.  So that provided the
motivation for the following demo.

As part of the preparation, I had also printed out skeletons of
different dinosaurs as well as Lucy and a modern human, and glued
these to pieces of cardboard.  I pulled out the stegosaurus and asked,
"Who knows what this is?"  Then we imagined stegosaurus caught in a
mudslide.  I had a volunteer help me pour the bucket of sand over the
stegosaurus (in the large clear plastic box).  Then, some time later,
here comes a...does anyone know what this is? Triceratops.
Triceratops dies and gets buried in a layer of wood chips, symbolizing
a different type of soil in that area at that time, which ultimately
forms a different layer of rock.  We repeated with a T. Rex and
another layer of sand.

Then we imagined that the area was underwater for a time.  We talked
about how an area could be underwater at times and above water at
other times.  We reviewed what they had learned about rivers and the
water cycle, and decided that layers of sediment can build up on the
lake's bottom or the sea floor.  We also related it to what they had
learned about the deep ocean, that things (like whale bones and
smaller bits of nutrients) rain down from above.  We simulated this by
having a few volunteers rain down black dirt, while I dropped an
elasmosaurus skeleton in.

Next, I did a special, thin, brightly colored layer using a bottle of
paprika.  They guessed it represented lava but I said we would come
back to discuss it later.

Then I brought out Lucy and discussed her, buried her in another layer
of wood chips and then brought out the modern human skeleton and
buried him in a final layer of sand.  The final product was
impressive, clearly showing seven different layers of "rock" through
the clear plastic.  (The box was about 2.5 feet long by 1.5 wide by
1.5 feet deep, and was about 2/3 filled by the end.)  We discussed how
the oldest rock layers are on the bottom and the newest are on the
top, so that the fossils we find on the bottom layers are of creatures
who lived long ago, and the fossils we find on the top layers are of
creatures who lived recently.  (This is true even if an earthquake
comes later and tilts the layers.  I tilted the box and asked who had
been to the Grand Canyon and seen the tilted layers there; a
substantial minority had seen it.)  Do we ever find cavemen (Lucy) on
the bottom layers? No.  Do we ever find dinosaurs on the top layers?
No.  We can even tell which dinosaurs lived earlier, and which lived
later.

Next, I had them exercise their hypothetico-deductive reasoning
skills.  If Lucy had lived as early as the dinosaurs, what would we
find?  If the dinosaurs had lived as late as Lucy, what would we find?

Finally, I returned to the thin paprika band. All over the world, we
find an easily identifiable band called the K-T boundary, and we find
dinosaur fossils only below that band, indicating that dinosaurs died
out around the time the band was formed.  And the band has been found
to contain an element, iridium, in much higher concentrations than
normally found on Earth, but consistent with a certain type of
asteroid.  The conclusion is that an asteroid impact and its aftermath
killed the dinosaurs.

I'm aware that this model is not universally accepted; some scientists
think volcanism played a role in the demise of the dinosaurs, and some
think the dinosaurs were dying out before the asteroid impact, which
perhaps only delivered the coup de grace. But there's only so much
detail you can go into with five-year-olds.  The best thing I can do
to help them deal with nuance as they grow more sophisticated is to
give them practice reasoning with evidence, just as I did.

I left the whole layer cake for the kids to excavate in their free time after lunch.
I had originally envisioned doing something which would make the layers set more
like stone so they would really have to chip away at it, but after finding out that
plaster of paris is toxic, decided not to go there.  I suppose a weak concrete might work,
and I may return to this idea in future years.  If I had done plaster or concrete, I would
have found something to color the layers slightly so they would show a bit of contrast.
As it happened, the sand/woodchips/black dirt made a beautiful set of layers.

I highly recommend reading this story of how Walter Alvarez and collaborators figured out the K-T boundary.  It really shows how
science works; it involves far more creativity and discovery than most
students are led to believe by being forced to do contrived lab
exercises in school.  Unfortunately, many K12 teachers have
experienced science only in that contrived, uninteresting context, and
themselves do not believe science requires creativity, and therefore
create a vicious cycle when they pass that attitude on to their
students.  I'll sign off with this link to a list of misconceptions about science.

Kindergarteners in Motion

Today at Primaria I did Newton's first and second laws of motion, much
as I did them at the elementary last fall.  If you read that entry, I
followed that plan up to and including how the Moon is attached to the
Earth by gravity.

Before doing that, I did a quick advertisement for the
tornado-in-a-bottle building activity I want to do next time.  We had
earlier, in the context of pressure, shown how water will not fall out
of a straw if you hold your finger over the top.  This is because for
the water to get out, air would have to get in to fill the space.  But
air can't get in when your finger is over the top.  So today I brought
in a giant size version of this: two 2-liter soda bottles screwed into
a connector, with one half-full of water.  Turn it upside down, and
the water doesn't fall because the air need to switch places but get
in each other's way. Now the cool part: swish the water around and it
forms a "tornado" which allows the air to go up through the center
while the water swirls down around the outside.

I bought plenty of the connectors and my goal is to collect enough
bottles to allow each kid to make one at school (with the option of
food coloring and glitter in the water!) and take it home.  Parents,
please bring in empty 2-liter bottles!

I bought the connectors (40 for $40) at teachersource.com.  I
recommend you try somewhere else because they took forever to ship,
and when I got them I found out that they leak.  I was able to prevent
leakage by wrapping the bottle threads with masking tape first, but I
shouldn't have to do that.  I went cheap because I wanted a large
quantity.  If you just want one or a few, Artec Educational has a
clear one (so you can see what's happening through the connector) for
$2 each.

Kindergarten Energy

Today I discussed energy with the pre-K/K kids.  I followed the same basic plan as I did when I discussed energy with the elementary kids (minus the last three paragraphs).  I know they've studied the water cycle quite a bit, so at the end I related it to the water cycle: water in the clouds has potential energy, water in the river has energy of motion, damming the river stores the energy, letting water our of the dam turns a turbine which generates electricity, etc.

In the remaining time, instead of having the kids draw pictures of different forms of energy as I did with the elementary kids, I let them play with the lights, prisms and lenses which were so popular last week. The primaria kids loved them too.  They were very disappointed when science time was up; in fact, one of them cried so much that I decided to leave all the materials at the school so that they could play with them in afterschool care as well.  This is a good thing for the future of science: girls crying for more science time!  Apparently this activity was a big hit in aftercare as well.

Under Pressure

The Primaria kids are learning about the ocean, starting from the deep
sea and moving up, so I decided to focus on pressure Friday.  I
started with a very simple giant syringe with the end capped, mounted
in a wooden block for stability.  I filled it with water and had each
of them press down as hard as they could.  Many of them already knew
(having encountered it when they studied the Marianas Trench) that
pressure at any point in the ocean is just the result of the weight of
the water above that point pressing down.  The syringe reminded them
of this point and made it vivid; they were playing the role of the
upper layers of water pressing down, and we imagined how much pressure
a creature would feel if it were in the syringe when they pressed on
it as hard as they could.  (Warning to future self: supervise more
closely because the syringe is easily broken!)

As a bit of an aside from the main focus of water pressure, we then
filled the syringe with air and repeated a round of pressing.  What's
different is that the air shrinks in response to the pressure!  The
plunger actually goes down when you press on it, and you can see that
that is not due to escaping air because the plunger springs right back
when you let go (assuming you have a good syringe where air really
does not leak around the plunger).  It's an interesting feeling to
feel the air pushing back like that; it really feels like squeezing an
invisible spring.  Water is very different: it pushes back without
changing its volume.  Physicists would say that air is compressible
and water is incompressible.

Next, we did the "three-hole can" experiment.  This is just a vertical
glass tube with three stoppered holes at different heights.  Fill with
water, and ask the kids to predict what will happen.  After they say
water will squirt out the holes, ask them if it will squirt out
equally fast (or far) from each hole.  You will probably get a variety
of opinions, at which point you can talk about the importance of doing
experiments to settle issues in science.  If there is a unanimous
prediction, I ask if we should still do the experiment, and we
conclude yes, because sometimes everybody is wrong.  This worked out
well Friday because one group had a unanimous prediction which was
correct, and then later had a unanimous prediction which was
incorrect, so they experienced both sides of it. 

In any case, if you unstopper all three, it's clear that water squirts
out fastest from the lowest hole.  This is because it is under more
pressure, having more weight of water above it.  (Technical note: if
you judge the pressure by how FAR the water travels before hitting the
ground, the lowest hole is at a bit of a disadvantage because its
water has less time before it hits the ground.  But that's a minor
factor for most setups.)

Next, I had set up a big tub of water with a stoppered hole at the
same height as one of the holes in the tube.  I restoppered the tube
when the water levels in the two containers matched, so the heights of
the water, as well as the heights of the hole, match.  Now the kids
have to decide, having removed the variable of height, which will
squirt out faster when unstoppered: the skinny tube or the massive
reservoir of water.  They turn out to be equal; pressure is determined
only by the height of the water above you, not by the volume.  A
practical application of this is that the strength required of a dam
is determined by the depth of the water it holds back, not by the
volume of the lake.

Next, we looked at how you can hold water in a straw by putting your
finger over the top.  This is related to pressure because if the water
started to fall out of the straw without air getting in, the air in
the straw would have to occupy more volume and thus be at lower
pressure.  The higher pressure of the outside air then provides an
upward push on the water to prevent it from falling.  (I simplified
this a bit for the kids; the summary for them was that the water could
not fall out without a way for air to get in to fill the space it
left.)  Then came the cool part; I have a similar setup in which air
is prevented from escaping from an inflated balloon, and it is amazing
to see how the balloon stays inflated even when you let go of the
neck!

Here's how it works. It start with a piece of glass which is shaped
more or less like an inflated balloon.  The exact shape doesn't
matter, but it needs a good amount of space inside for the balloon to
inflate, and it also needs a neck on which to mount the balloon's
neck.  Stuff most of the limp balloon inside, and mount the balloon's
neck on the glass container's neck.  Then inflate the balloon, which
surreptitiously pushes air out of the glass container through a hole
in its back side.  Then insert a stopper into that hole, and there's
no way the balloon can deflate; in order to do so, you would need to
remove the stopper so air can take up the space in the glass
container.  So take your mouth off the balloon's neck and watch the
kids gape...its neck is held wide open by the glass neck, but the
balloon does not deflate!

You can then ask them to predict what will happen when you remove the
cork, and there are further variations such as pouring water in the
balloon before removing the stopper (which creates a nice squirt of
water when you do remove the stopper). 

I found that the whole thing went pretty quickly, and there would have
been time to also do the vortex bottle, which I will write about next
time.  Some groups had extra time to blow up balloons and release
them, and do other hands-on experiments with the equipment, but that's
always a good thing.

I also (re)discovered something about managing the kids.  I had them
all sit around a big round table, and that eliminated a lot of the
annoyances which had wasted time in previous activities: kids
jockeying for better position, kids getting distracted by the
playground equipment (we were outdoors for obvious reasons), etc.  In
the future I should set things up so that they are seated if at all
possible.

A simple, fun extension of this activity you might want to do at home is to
make a giant version of the three-hole can.  Stand an 8-foot PVC pipe
straight up, make the bottom end watertight, drill a bunch of holes in it,
and put a hose in the top.  This will make a good thought-provoking
sprinkler for the summer!

Seeing the World in a New Light

On Friday I was back at Primaria, building on
what we learned last time about colors of light. I handed out diffraction
gratings again, so they could again see how white light is composed of colors.
(I have my doubts that they really get the message that white light is
composed of colors; what they take away seems to be that the device
makes rainbows appear, which is not at all the same thing.  Maybe a
dark room with a single source of light would help.)  I asked them to
list the colors in order of appearance, in English and Spanish.  I was
surprised to see how much trouble they had listing them in order of
appearance on the diffraction grating; they were all over the map. One
idea for the future is to have them draw what they see.  But I was
even more surprised when they burst out in song "Red, orange, yellow /
Green, blue purple" when they realized that I was essentially asking
for the colors of the rainbow.  They had memorized this song which
perfectly described what they were unable to describe when asked about
their observations!  There must be something I can learn from this,
but I'm not sure what.

I wrote the names of the colors in this order on the board, so that
"range of colors" had a more concrete meaning. Then I asked them if
there could be anything "before" red or "after" purple (aka violet).
One boy in one group actually guessed infrared, presumably because he
had overheard me use that word to a teacher.  I briefly mentioned
ultraviolet to connect it with the need for sunscreen in summer, but
the main purpose was to show the infrared camera.  Things at room
temperature shine in infrared light, and the warmer they are the more
they shine, so the infrared camera is really a very different way of
looking at the world.  You can walk into a kitchen, as we did, and be
blinded by the water boiling on the stove.  A hand holding an ice cube
looks like a bright silhouette with a black rectangle in front of it.
People glow...you can see the glow radiating out from the openings in
their clothing, and from under their hair.  You can make your way in
the dark, because everything is glowing to some extent.  You can find
people hiding in the dark.

One group insisted they could see in the dark with their own eyes, so
I took them into the only room in the school which has no window: the
adult bathroom.  (Linus had previously voiced this "I can see in the
dark" meme with great confidence, so I was sure it was something that
kid in the school bragged about and needed to be refuted.)  I was
pretty sure that when we got in there and shut the door, most kids
would be scared and admit that they couldn't see in the dark.  That
happened to one child, but several others were perfectly content with
the light coming from the gap under the door and said, see, we can see
in the dark.  Preconceptions are very difficult to uproot!  I
discussed this with Linus later and we agreed that he could see when
it was kind of dark, but not when it was perfectly dark.  So
terminology matters.  Given a way to save face, I got him to agree
with my scientific conclusion; but I had to give him a way to save
face.  In any case, the little bit we could see in the mostly-dark
bathroom was nothing compared to how well we could see with the
infrared camera!

I also showed how the IR camera could see through some things which
are opaque to visible light (eg, a black plastic garbage bag, and I
mentioned but did not demonstrate smoke), and others are opaque to IR
but transparent in visible light (eg, glass windows; when I pointed
the camera at the window we saw only my reflection on the camera
display).  Outside, the sky is black even during the day because the
air is relatively cold.

If all this sounds interesting, I recommend you watch this video to
get an idea of what the world looks like in the infrared.  Meanwhile,
I have to make some notes for future demos: (1) it was hard for
everyone even in a group of seven to see the display.  I thought I had
solved this problem by bringing a laptop and not relying on just the
camera's small screen, but you cannot imagine how seven kids have
trouble seeing the same laptop screen at once.  In addition, the latop
screen was small enough that it didn't really grab their
attention. Next time, I will either bring a large LCD display, or use
a projector.  (2) This activity is not very hands-on.  The kids had
trouble staying tuned in.  It would be good if I had the camera on a
fixed tripod out of their reach, set up a giant display with the
projector, and just let them do things and bring things in front of
the camera.

Let There Be Light! Part II

You may recall that I was disappointed with the way my light and shadow session with Primaria just before Christmas break turned out,
so I decided to cover some of the same concepts again with a different
approach, as well as add some new concepts related to light.  I bought
some powerful light sources from Big 5: an LED lantern and a big
LED flashlight, and I borrowed an old slide projector just to use as a
light source.  (Note to self: check eBay for used slide projectors!)

I started by showing how white light is made up of many colors.  I had
them look at a compact fluorescent bulb through a special piece of
plastic (a diffraction grating) which separates the different
wavelengths of light.  Different wavelengths are perceived as
different colors, so they saw this:

The compact fluorescent bulb is at left.  I stuck a diffraction grating in front of the camera lens, which separates the wavelengths (colors) of light so that we see a purple bulb, a blue bulb, a cyan bulb, a green bulb, and orange bulb, and a red bulb off to the right.  This shows that what we perceive as white light is actually composed of a mixture of colors.

They described it as a rainbow, and yes, it does show that white light
is composed of colors, but it's actually even more interesting than
that.  (This part in parentheses I did not go over with the kids, but
it might be useful for other audiences.  If the light bulb emitted a
continuous range of wavelengths, the colors would be spread out
continuously and would just be a smear like an actual rainbow is.
This is what an incandescent bulb would look like through a diffraction grating.
But the CF bulb emits only a few specific wavelengths, so you see a
well-defined image of the bulb in one very specific shade of purple,
another well-defined image in a very specific shade of blue, etc.
This is related to how the CF bulb gets its greater energy efficiency.
The incandescent bulb emits a very broad range of wavelengths, from
the ultraviolet to the infrared, and many of these are wasted because
human eyes cannot detect them.  The CF bulb by design emits only a few
specific wavelengths which are all chosen to be visible, so none of
its output is wasted.  Therefore, for a given amount of visible light
output (listed in lumens on the package), it uses much less energy
(listed as watts on the package).  So always buy light bulbs by
looking at the lumens, not watts!  You may also see CF packages which
say "100 W equivalent", which means they give off as many lumens as a
100 W incandescent bulb, even while they drain fewer watts from the
grid.  And this is also why the light from a CF bulb is sometimes
ugly: engineers have to work hard to build a set of specific
wavelengths which convincingly fake a full range of wavelengths.)

We did talk about what colors might exist beyond violet on one end of
the spectrum and beyond red on the other end.  I asked, and none of
them knew that ultraviolet light from the Sun can cause sunburn, so
now they know why their parents slather on sunblock so often!  I also
mentioned that next time I will bring an infrared camera so they can
see what the world looks like in that "color."

Having established that white light is composed of many colors, I
asked them about their experience mixing paints of different colors.
What do you get when you mix many colors of paint together?  Some of
them knew that you get a yucky dark mess, not white.  So mixing colors
is different with light than with paint!  (Again, this is above their
age range, but possibly useful to readers: the difference stems from
the fact that red paint, for example, is red because it absorbs all
colors other than red, and reflects red.  So if you mix that with
paint which absorbs all colors other than green and paint which
absorbs all colors other than blue, you basically have paint which
absorbs all colors!  Paint basically subtracts light rather than adds
light.)

Next, I fired up the slide projector, which I has set up with a small
statute near the screen so that the statue's shadow was clearly
visible.  (In practice, I had determined that using a 5-year-old as
the shadowmaker was not practical; it was difficult to keep him still
and to give directions about moving right vs left, etc.  I also
darkened the room as much as I could beforehand.)  I then turned on a
powerful flashlight and showed how you get two shadows with two light
sources.  (This is a powerful argument against Moon-landing-hoax
conspiracy theorists, by the way, who claim that the lighting in the
astronaut's videos came from multiple studio lights; look at the video
and you will always see only one shadow from each object.)  I moved
the flashlight around to different places, asking them to predict what
would happen to the shadow (where would it move?  would it get taller
or shorter?) each time.  This sounds very simple, but it took a few
iterations before they got it.  I think this topic was right at their
level.

Next, I turned off the flashlight and put a yellow plastic film in
front of the projector.  We saw that the shadow was still black,
because no light was getting there.  Next, I turned on the white-light
flashlight and we saw that one shadow was yellow and the other was
white!  That's because one shadow resulted from the statue blocking
the white light, and the other resulted from the statue blocking the
yellow light.  Then they were eager to put another color plastic film
over the flashlight, say red to start with.  I was careful to do this
in stages.  I didn't mix the red flashlight light with the yellow
projector light until they had made a prediction.  Then, I pointed the
flashlight somewhat away from the statue so that we saw how the light
mixed without thinking about the complication of the shadow.  Then we
did the two-color shadows.  Then we had a blast trying other color
combinations.

In each group, one person asked if we could put the red plastic and
the yellow plastic in front of the projector.  First we discussed how
the plastic blocks all the colors in white light and only lets the
yellow through, so if I then put a piece of plastic which blocks all
colors other than red I should get what?  Few of them got it, that it
would block almost all the light (the little light that did get
through appeared orange; exactly which shade depends on the details of
exactly how efficiently each film blocks each wavelength).

But that's not all!  As more of a gee-whiz demo rather than teaching
principles of physical science, I had brought glow-in-the-dark paper.
I was not able to get the room very dark, so it was
unimpressive...until I whipped out a purple laser and started writing
on it with light!  The laser is powerful enough that the spot where it
hits the paper really glows, and the bright glow lasts for several
seconds, so I could write a child's name and finish just as the first
letter was vanishing.  This was a nice connection to other parts of
their curriculum: they were studying the deep sea and had just learned
about bioluminescence, and also they are learning their letters so
many of them could recognize their own names, and were extremely
gratified to recognize it when I wrote it.

For the groups which had a few extra minutes, I let them play with
either the slide projector and color films, or colored Christmas tree
lights, which make it easy to mix colors (warning: with the newer LED
lights the mixing is not that impressive because of the
specific-wavelength vs range-of-wavelengths issue discussed above).
In the last group, two kids stayed an extra 20 minutes while I packed
up and their peers were playing on the playground!  So it seems to
have been interesting to them.

Diffraction gratings can be ordered from Edmund Scientific for about
$1 each, and I just ordered some for the school so kids can experiment
outside the very limited hours I am there.  If you want to do more at
home with your child, I suggest getting a few strongish flashlights,
taping different color plastic films over them, and having fund after
the Sun sets.

Let there be light!

Back to Primaria (pre-K/K) this week.  The teachers asked me to
explain how lenses work, because the kids had been making toy
eyeglasses out of pipecleaners and were curious about it.  I had long
wanted to do some demos with light anyway.  It takes a lot of trouble
to make a room really dark (so that the light relevant to the
demonstration is more visible) during school hours, so I figured I
would go to that trouble and combine topics.  Linus (my son in
Primaria) had asked just a week or so before about the Moon.  He
thought the phases of the Moon were due to Earth's shadow falling on
the Moon.  I pointed out that the crescent Moon appears not too far
from the Sun, so the Earth's shadow cannot be falling on it.  He came
up with some crazy stuff about light bouncing back and forth, back and
forth between Earth, Sun, and Moon "like an air hockey puck."  So I
had a motivation to do phases of the Moon with the kids, but first I
had to build on basic concepts of light, like the difference between
emission and reflection (the Sun emits light and is the source of
light in our solar system; the Moon reflects some fraction of the
light it receives, but not enough to illuminate the other bodies in
the solar system).

So I set up in the kids' bathroom, which is the only room with no
windows.  I still had to spend a lot of time taping up the open
doorway with black plastic to prevent a lot of light coming in.  In
groups of 5-6, the kids came in and we started by talking about how we
couldn't see anything without a source of light.  I then turned on an
unexpected source of light: a laser pointer.  We discussed how they
couldn't see the source of light directly, but they could see the
light when it reflected off the ceiling. Next, a flashlight.  I
pointed it directly at them, then pointed it at the ceiling.  So a
given light source can be seen directly or indirectly (reflected)
depending on your relationship to it.

Now I turned on the "Sun": a naked light bulb.  Unlike a flashlight or
laser pointer, it emits in all directions.  But can we always see the
Sun?  We discussed various reasons for not seeing the Sun, such as
clouds.  But when is it really dark?  At night.  And what is night?
"Clouds" were again offered as a reason, so we discussed what happens
just before night: "the Sun goes down behind the mountains."  So then
we each pretended we were the Earth, and slowly turned around so that
the Sun came into and out of our field of view.  [The next day, my
wife Vera offered a really good suggestion: have them extend their
arms to make a "horizon" which turns with them.]  To be honest, a lot
of kids spun way too rapidly and weren't really getting it.  I
repeated the whole thing with a globe.  We agreed on the location of
California and looked at how California varied between bright and dark
as the Earth turned.  A problem with this is that light reflecting off
the walls provides a non-negligible amount of illumination for the
back side of the Earth, and the effect is not nearly as dramatic as
you would thing.  Vera suggests decoupling the day/night concept from
the light demo, just pasting up a picture of the Sun in a regular
classroom and doing the horizon thing.  I think she's right about
that! Another possibility is to build a little model.  If the Sun were
a Christmas-tree bulb and the Earth a nearby marble, relatively little
light would bounce off the walls of the room!

Next, we tackled phases of the Moon.  I had one child volunteer to be
Earth while I took a volleyball Moon and moved it around Earth,
showing how the Earth-person sees a fully-illuminated Moon when it is
opposite the Sun, and sees (rather, does not see) an un-illuminated
Moon when it is more or less between Earth and Sun.  However, this did
not work well for several reasons.  Each group had a bunch of other
kids who were not the Earth and saw the whole thing from a variety of
vantage points.  It was very difficult to steer the kids into seeing
what they were "supposed" to see.  One girl said "now I'm the Earth"
when the Moon happened to come close to her.  In one group, the
Earth-volunteer gave the wrong answer when I asked him whether the
side of the Moon he was seeing was bright or dark; I think he just
didn't know what to compare to, so I need to be more careful about
exactly how I word my questions.

Finally, the lens.  The key to a good visualization is to avoid using
all three dimensions. I put a flashlight on a table so they can see
how the light spreads out by looking at the light and dark patterns on
the table.  I put a comb in front of the light to give a visual
impression of light rays spreading out on the table.  Then I set a
special lens on the table, which is like a slice of a lens so that it
can sit flat on the table.  This shows that the light rays which go
through the lens are bent so that they converge back together rather
than continue diverging.  It's quite striking if set up right.  I had
a card which I pretended was a movie screen, and projected the focused
image there. We talked about movie theaters and where they would sit,
did they ever look behind them and see the bright light coming out of
the lens, and what would happen if there was no screen.  For the
groups which had a bit of time left at the end, I moved the lens
around to show that if it's too close to the light, it's not powerful
enough to converge the light.  It might be powerful enough to stop
further spreading of the light, though, and I showed how a second lens
could then converge that light.  The idea was to show that there are
many combinations and possibilities.

I felt that the kids were more disengaged than usual, and I felt that
it was directly attributable to the "demo" rather than "hands-on"
nature of the activity.  I made the "demo" decision because I felt it
would be chaos to have 4- and 5-year-olds handling flashlights and
lenses in teams of two or three.  That may have been correct, but I
should have found some way to prevent the whole 20 minutes from being
all demo.  One possible structure is the sandwich: an initial demo
followed by hands-on activities with a more complicated demo or
summary discussion at the end.  But this didn't fit with the list of
topics I wanted to cover.  I realize now that I was too much in
"professor" mode: practicing inquiry is more important than covering a
list of topics!  The kids brought up (indirectly) one thing I had
thought about last year but forgotten: setting up a light so that they
can make shadows themselves.  They love doing this, and if I set it up
right they can actually explore different aspects of light.  For
example, I could set up two lights of different colors and they could
see how to control the color of the shadow.  Or they could explore how
a given object can have differently shaped shadows depending on its
orientation to the light.  I can even imagine setting up a "light
studio" which they could play with during the week before or after my
visit.

Gearheads

The Primaria kids continue to be fascinated by contraptions,
factories, and the like.  Thursday, the day before my visit, they
built contraptions using empty cardboard boxes, egg cartons, steel
cans, etc, plus a lot of imagination.  So I explored pulleys and gears
with them on Friday.

In my previous visit we built an elevator; that was more about the
principle of balance than about pulleys, but it did give them a basic
intro to pulleys. This time, I rigged up different pulley arrangements
to lift identical concrete blocks, using the monkey bars to hang the
pulleys.  One arrangement was just a single pulley at the top as you
might expect.  The second arrangement had the end of the rope tied at
the top, running down to an "upside down" pulley attached to the
block, and then back up to a pulley at the top which acted much like
the single pulley, just reversing the direction of the rope so that
the kids could stand on the ground and pull down on the rope to make
the block go up.  The kids tried both setups and compared the
difficulty of lifting the block.

The second arrangement is much easier.  I didn't expect the kids to
figure out why, but I did expect them to see that it had two pulleys
instead of one, or that it had a moving pulley rather than just a
fixed one.  Two of the four groups did not see this and required some
coaxing.  But I made a kind of game out of it, telling them that in
science we have to be very observant, asking them to watch carefully
as I pull each one slowly, etc.  I was happy to be able to frame it
such that they could gradually work toward the answer rather than just
have me give them the answer.

So why does the moving-pulley system make it easier?  I took lots of
very entertaining guesses on this one before having them observe the
motion again.  The moving pulley makes it so that if I pull my end of
the rope one foot, the weight moves up half a foot.  This means that
you only need half the muscle that you need with the fixed pulley.
(This is called "mechanical advantage" but I did not use that term.)
Then I asked how they could imagine making it even easier to pull.
Some of the groups digressed at first ("add a motor", "get a lighter
block") but we generally concluded that even more pulleys would be
better.  Kids love big numbers, and instead of suggesting four pulleys
some went straight to "a thousand pulleys!"  I had tried setting up
four pulleys, but the ropes got too twisted.  If you want to go to
four pulleys, I recommend buying sets of two pulleys already bolted
together side-by-side to avoid this twisting (parallel pulleys).  I
can't imagine how twisted the ropes would get with a thousand pulleys!

Next, we did gears.  They had already played a lot with
Gears!Gears!Gears! sets, but those are limited in terms of gear
concepts.  I ordered some bags of gears of very different sizes and
had hoped to mount them in some way which allowed for exploration, but
I ran out of time drilling holes at 8:45 Friday morning.  So this was
more of a demonstration than a hands-on activity, but that was ok
because it allowed me to use something I had only one copy of: the
book Get in Gear by Sholly Fisch, which is a really nice book.  Each
page describes a new gear concept and gives you the framework for
assembling it and seeing it work for yourself.

Before going to the book, I wanted to make sure they understood gear
ratios (although I didn't use that term).  I showed a little gear
turning a big gear in one of my homemade setups, and we counted how
many times we had to turn the little gear all the way around before
the big gear went around once.  In this case, it was about 3, because
the big gear had about 3 times as many teeth.  Conversely, turning the
big gear once makes the little gear go around about 3 times.  So if
you need to build a high-speed machine, hook a motor up to a big gear
which turns a little gear, and the little gear will go crazy fast.
And if you need to build a low-speed machine, hook your motor up to
the little gear, and the big gear will trun slowly.  We talked about
why people might need to build a low-speed machine.  This connects
back to the pulleys: when moving a heavy weight, low-speed is
better. (I left it at that without talking about forces; I think the
low-speed motion of the concrete block in the easy-to-pull setup was
the most effective and appropriate "proof" for this age group.)

On to the book.  I had noticed that the kids are paying attention to
clocks and starting to learn about time, so I started with the clock.
This was a natural segue from the gear ratio demo.  We want to make
the hour hand go slowly, so how do we do that with gears?  Attach the
hand to a big gear which is driven by a small gear!  And we want to
make the minute hand go fast, so how do we do that with gears?  Attach
it to a small gear which is driven by a big gear!  I was pleased that
the kids were able to guess these answers most of the time.  So here's
the clock in action:



Next, I showed them that gears are not limited to circular motion. Here is a rack gear in action:

Rack gears are used for turning circular motion into linear motion.  In addition to all kinds of machines, rack gears are used in steep mountain railways, where the track contains the rack gear and the engine carries and pushes on the circular gear.  (It's also used for rack-and-pinion steering; the pinion is the circular gear which meshes with the rack.)  We talked about what kinds of machines might need to do this kind of motion. Maybe squeezing grapes for grape juice, or printing presses.

And we can also set up gears to do a sweeping motion, by attaching something off-center:

The last thing we had time for was planetary gears, so called because little gears go around a bigger gear like the planets around the sun:

This is cool and could just be a work of art, but there are applications. Note that around the outside is what is basically a really big inside-out gear (difficult to see in the video because it's made of clear plastic). I went back to the homemade big+small gear setup and asked why it would be useful to put the small gear inside the big gear. Answer: to save space, if you need to make a small machine, like a pencil sharpener, a kitchen mixer, maybe an electric toothbrush.

Finally, we didn't get time to build the piece de resistance, but here is a machine which combs your hair and brushes your teeth at the same time:

The whole activity worked well.  I learned something about organizing kids, too.  Because there weren't enough pulley setups, kids had to wait, but there wasn't really a line because it was just a few kids waiting.  This led to a lot of confusion until Teacher Jessica brought "waiting chairs."  When the kids have to sit in chairs to wait, it is 100% clear who is next! 


If you want to see more, I recommend the video Gear Basics.

Going up?

The pre-K/K kids have been really interested in machines for a few
weeks now. When I first heard about that interest, I (with Linus's
permission) brought our set of Gears!Gears!Gears! to the room for a
long-term loan.  Since then, I have seen kids playing with the gears
every morning I drop Linus off.  When we saw a slightly more advanced
set of Gears!Gears!Gears! in Costco one Sunday (with different size
gears and a loop gear, plus some non-gear bells and whistles), it was
a no-brainer to buy that and bring that for a long-term loan as well.
The kids seem to really be into it.

So I thought of building on that interest by doing something with
pulleys, and I settled on building a simple elevator as an activity
which seemed doable, but still challenging enough to be interesting.
I borrowed a big old pulley from the physics department, brought some
of my own ropes and weights, and counted on the school having some big
dairy cartons and a decent place to hang the pulley.

After some looking around and testing, I settled on a certain tree
branch as a good place to hang the pulley, and I found a dairy carton
big enough for a kid to climb into.  With the first group, I started
from scratch, asking them what they thought would be necessary to
build an elevator, and they suggested a basket (they even found one)
and rope (which I supplied).  They needed a bit of prodding to suggest
a pulley, but they got that too after I suggested looking above my
head.  Most of them didn't really know what a pulley was, so we
discussed that.  I strung a rope through it and we each verified that
pulling down on one end of the rope made the other end go up.  The kid
were really excited at this point!  It was difficult for some of them
not to grab the rope, jump up and down, etc.  I pointed out that one
advantage of the pulley is that the puller (the kids in this case, a
motor in real life) need not be on the roof to make the elevator work.

Then I attached the large milk carton and put some heavy object in it
for a first test.  The more excited kids volunteered to pull on the
other end of the rope.  They were able to lift the elevator, but it
was quite difficult; they had to recruit help and I think it was
successful only with four boys pulling at once.  I warned them that if
they let go suddenly, the elevator would crash to the ground and hurt
the (imaginary) people in the elevator.

So I asked them to think about what could make the pulling and the
letting down easier and safer.  They thought of all kinds of crazy
ideas before they spotted my weights.  So I attached the
counterweights (in a small basket so we would adjust the amount of
counterweight) and we saw that the elevator was much easier to lift
and also easier and safer to let down.  So then we were ready to give
rides.

The problem was that the dairy carton tilted too easily when lifted
off the ground, threatening to dump the passenger out.  I tried to
stabilize it with additional ropes and by telling the passenger to balance,
but it never really worked.  So starting with the second group, I
forbade rides.  Instead, we used three containers full of sand to
represent three people.  This was actually nice for the lesson because
I was able to put in just enough sand to balance the particular
counterweight I had; with a human passenger, the counterweight was a
help, but never really made it super easy to ascend and descend.  With the fake
passengers matched to the counterweight, ascents and descents were very easy,
and I could tell the "motor" to let go, simulating a broken motor.  The elevator
did not crash to the ground because it was attached to the just-right
counterweight.

So, once we got it going smoothly, I repeated these steps for each
kid: remove the counterweight; ask them to lift the passengers to the
top floor and have them discover how difficult that is; have them feel
how tricky the descent (from whatever point they reached) is; after
finishing the descent, add the counterweight and ask them to lift the
passengers to the top floor and see how easy it is this time; ask them
to let the passengers descend safely and feel how easy that is; ask the
motor to "break" and see how the passengers do not crash to the ground
because of the counterweight; finish the descent and start over with
another kid.  I repeated this whole cycle about a million times
because many kids wanted to do it over and over!  I was exhausted by
the end.

This was a pretty simple activity and the kids had a lot of fun.  This
is a good lesson for me because I'm often tempted to think that a
potential activity is too simple and that I have to add a lot to it.
Simple can be good!  If I ever try rides again, I need to experiment
beforehand how to make the elevator "car" tip-proof.  But I think the
rides may have been a distraction.  Each child was quite happy in the
"motor" role, so much that they wanted turns over and over, and of
course the motor role is the instructive one.

A small improvement would be to use two pulleys, to give some
horizontal space between the elevator car and the counterweight.  One
thing which would take this to the next level would be to crank the
whole thing with some gears attached to a drum which winds up the
rope.  I'll keep my eye out for surplus equipment which might be used
for this.  And for a toy gear set with these kinds of pieces, which I
will then have to buy and put on long-term loan!

A boat which sinks on purpose

This builds very well on the previous pre-K/K activity, in which we
investigated what floats and what sinks.  By the end of that
activity, the kids had figured out that sinking a plastic soda bottle
takes quite a bit of effort.  They need to put in some heavy things
like rocks (small ones which fit through the neck), but that is not
enough; they also had to get rid of most of the air by replacing it with
water.  We started by reviewing what we had learned last time (by me
asking them questions, not by me lecturing).

After reviewing the basics, I showed them a bottle with some rocks in
it and the cap on and asked them if it would float or sink.  Many of
them had forgotten how easily it floats unless it's really full of
rocks.  We talked about boats, how they float because they have lots
of air inside (we had made aluminum-foil boats last time), and how a
submarine is special because it has to sink when desired but then has
to float again when desired.  Most kids are under the misconception
that submarines dive just by having their engines push them down, but
if so then subs would have to have engines roaring just to stay still
underwater.  Instead, they really do sink.  We talked about how the
rocks inside represent heavy stuff that has to be on the submarine,
like engines and other equipment, people, etc, and also how there
still has to be some air on the submarine for people to walk around
and breathe.

So I challenged them to make a bottle almost sink with heavy stuff,
and then I would help them with the next step.  I brought a box of
small rocks, and one bottle per child.  They consistently
underestimate how many rocks it takes to sink the bottle with the cap
on.  It needs to be about 2/3 full (although a fair fraction of this
2/3 is still air pockets between the rocks).  When a child was ready
test, I put on the cap for them and tested it even when I knew it
wouldn't sink.  Eventually they got close enough, but next time I
might consider marbles or something similar which would roll into the
mouth of the bottle more easily than irregularly shaped rocks, some of
which were too big anyway.  The kids got a lot of practice making
predictions that it would sink, testing those predictions, and
modifying their hypotheses.

Before class, I had drilled two small holes in each bottle so water
could enter and exit if desired.  These were drilled along one side,
which is considered the bottom of the sub when the bottle is floating
lengthwise, somewhat resembling the actual shape of a sub:

With enough ballast to get it close to sinking, I gave the kids new
caps which had had holes drilled and straws inserted through the holes
in a (nearly) watertight manner.  It worked well that each kid got the
ballast done at different times, so that I could do some one-on-one
with each at the critical moment.  I pointed out how the darn thing
still wouldn't sink, and why do they think it wants to float so much?
We would eventually hit on the idea of getting rid of the air using
the straw. Many of the kids were not old enough to know the difference
between blowing and sucking!  They were supposed to suck the air out,
thus pulling water in through the holes on the bottom.  Many blew to
begin with, but figured it out.

When they finally got it to sink, it was cause for high-fives.  I made sure to
point out that there was still air in the sunken sub, so the crew would still be
able to breathe.   I then challenged them to get it to float again, which involves
blowing on the straw, thus forcing water out through the bottom holes.
From that point on, it was just fun time as kids experimented with
their creations.

I think we took about 20 minutes per group of five students.  I would
recommend using a shallow container of water such as a water table,
not an aquarium!  Water deeper than say 6 inches is just unnecessary
and a pain....straws are only so long, and high aquarium walls make it
difficult to reach.

This activity was pretty successful in terms of student interest.  The
first group was the most difficult, because they had to put in all the
ballast, which was a lot of work.  After that, I took out just some of
the ballast between groups so that each group went through the process
without it being quite so arduous.  As I wrote above, in the future I
should check out types of ballast which will go in more easily.

Floating and Sinking

Most kids love playing with water, and in hot weather water is a good thing to do science outdoors with. (Not to mention that the ocean is the theme in Primaria this year!) Discovering what sinks and what floats is a natural entry point for science because it is so simple that the youngest kids can appreciate it, yet it can lead to quite sophisticated concepts for the older ones who are ready to handle those.  Furthermore, I designed this activity to lead naturally up to a submarine-building activity I want to do next time.

I started with just a simple glass of water visible.  I asked each
child if they thought a wood chip would float or sink.  For me, this
next step is really important.  If the vote is not unanimous, I ask if
we can settle the issue just by counting the votes.  Science is not a
democracy!  We have to do the experiment and pay attention to the
results if we want to make any progress!  And if the vote is unanimous, I
ask them if maybe we don't need to do the experiment.  We agree
(sometimes with some nudging from me) that even if we all think it's
going to float, we should still do the experiment because sometimes we
could all be wrong in our predictions.  I really want to emphasize
these aspects of the scientific method as early as possible, and this
activity is a good place to do it.

Then I repeat with several objects, such as a stone, a marble, a piece
of plastic, a bolt, a paper clip, etc.  The kids have some idea that
lighter things are more likely to float, so the paper clip gives some
pause.  I try not to use the word "density" because this means nothing
to the pre-K/K kids, but I do try to summarize that floating/sinking
is expected for something that is light/heavy for its size, not just
light/heavy in some absolute sense. 

Then we get to the more interesting demo.  (Some of them desperately
want to play with this stuff already, but I promise they can play if
they pay attention for just a bit longer.)  I pull out a hard-boiled
egg and we see that it sinks.  But if I add plenty of salt to the
water, the egg begins to float.  This shows that the salt is mixing
with the water in a way which makes the water heavier.  (By the way,
floating an egg is apparently how people used to determine they had
added enough salt to their pickling solution when making pickles.)
Then we repeat the whole thing with sand.  Try as we might, the egg
does not float and the sand just collects at the bottom rather than
dissolving in the water.  Here we have the observational basis for
some chemistry: salt in water forms a solution, but sand in water does
not.  (I didn't state it this technically, but we did talk about how
ocean water behaves at the beach...the salt is an integral part of it,
as we can tell by its taste, but the sand is not.)  We also have the
idea of different kinds of mixtures, which ties in nicely with the
previous pre-K/K science activity.

Finally we get to the play time.  But this is serious play.  I bring
out one tub of water in which I place some aluminum-foil boats.
Although they are metal, they do not sink.  I challenge them to figure
out how to sink the boats. In parallel, a second tub contains empty
8-oz plastic soda bottles which I also challenge the children to sink.
The challenge aspect is really important.  They come up with the ideas
and try them out. It seems like play time, but it has a purpose.  This
particular challenge has the extra purpose that it builds up to the
future submarine activity.

With the foil, I have extra challenges ready for those who quickly
figure out how to sink the boats with stones.  I challenge them to sink
the foil just by crumpling it up into a ball.  It is surprisingly
difficult to do this; small air bubbles trapped in the foil are
surprisingly effective at floating it even after squeezing as hard as
possible.  Some of them easily recognize that air bubbles must be the
problem, while others need some hints.  The persistent ones finally
succeed in hammering out the air bubbles using anything vaguely
hammer-like.  Meanwhile, others have gone in a slightly different
direction, crumpling the foil around a stone so that it forms a ball
with high average density.

With the plastic bottles, students take one of two initial strategies:
filling the bottles with water, or with stones/sand.  Those who try
water see that water is not heavier than water, so that a waterlogged
plastic bottle still does not sink. Then they tend to start over with
stones/sand.  However, the stone/sand strategy is surprisingly
ineffective.  You can fill a bottle 1/4 full or even 1/2 or even 2/3 full of
stones/sand and it still doesn't sink.  There's just too much air in
the bottle.  However, few students have the patience (or the time left
in the activity) to fill the small-necked bottle completely with stones/sand.
They figure out (possibly with some hints) that they can
replace the bothersome air with water and finally get it to sink.
This is really good background for the submarine activity!

I think we spent 20 minutes with each group of about 5 kids, and that
was the perfect amount of time and the perfect size group.  Larger groups could be
accommodated with more tubs of water; more than 3 kids per tub would not be good.

The Sift Hits the Sand

Today was my first day with the 4-6 year olds.  I generally try to
think of an activity which builds on or is related to what the kids
are doing the rest of the week, so that my visits are not put in a
pigeonhole marked "Science" which has nothing to do with the rest of
their lives or studies.  (A big point I want to get across is that
Everything is Connected.  At the university level this might mean
emphasizing the unity of knowledge---students tend to see different
chapters of a textbook or different lectures as unrelated pigeonholes,
and must be prodded to think about the connections, which are actually
the important part!  But for these kids, it's enough to make
connections between science and their everyday lives.)

But this being the start of the school year, the emphasis so far has
been on community-building, and there wasn't an obvious hook into
physical science.  Teacher Jessica said that the kids had been
fascinated with some aspects of sand, so I thought of a way to build
on that.  I had them separate big, medium, and small particles from
the sandpile, and used that to discuss solids, liquids, and molecules,
as well as engineering.

Simple materials.

Before class, I built seven sets (seven is the maximum number of kids
per group) of coarse and fine sifters at low cost as follows.  I took
a 4" diameter PVC pipe and sliced it into short segments to form the
frames of the sifters.  For the mesh, I bought screen material.  I
wanted a variety of mesh sizes, but this was difficult at the hardware
store.  I ended up using what is basically window screen material.  I
also had on hand a much coarser wire mesh designed to form a skeleton
for papier mache constructions.  So I had two sizes, although I would
have liked even more and I will keep my eye out for different
materials in the future.  (A baker's sifter has a finer mesh, but mine
had no walls so it was too easy to spill the sane rather than sift the
sand.)  I cut the meshes into circles and duct-taped them onto the PVC
frames.  I also brought some small cardboard boxes, some paper coffee
filters, and 21 (3 for each child) 44-oz plastic cups, which happen to
have mouths which fit well with the 4" PVC pipe.  I wanted to bring
tweezers as well, but I forgot it.

I showed each group that I had been able (before class) to obtain one
cup of big stones and woodchips, one of medium stones, and one of fine
sand, and I gave them 10 minutes or so to experiment with any and all
of these tools to see if they could do it.  They all pretty much got
it, usually with some guidance (as much to keep them focused as to
show them how to do it), and no one found it so easy as to be boring.
One of the girls found an advanced way to do it: stack the coarse
filter on top of the fine filter on top of a cup, load the top with
sand, and shake the whole thing to do it all at once.  Like an oil
refinery, but with the heavy stuff staying on top!  This is why I
mentioned engineering: although I often emphasize the cognitive value
in being able to understand or accomplish something in more than one
way, there is often great practical value in finding the most
efficient way!

The oil-refinery configuration with the finer mesh in the middle. If we had more types of mesh, we could separate into many different sizes all at once.  Chaining together many separation devices to derive an ultrapure sample is a principle used in a variety of contexts I did not discuss with these kids, such as uranium enrichment. They might be able to see that a coin-sorting machine might be built this way, though.

We then talked about alternative ways to do the separation.  Some had
wanted tweezers to separate the particles one by one; I forgot to
bring tweezers, but that's a valid---even if very
time-consuming!---way to do it.  No one thought of using the box, but
when I asked how they would use the box about one kid in each group
guessed that if I just shake a box full of this mixture, the bigger
pieces come to the top.  I even brought a cereal box to make the
connection to every kid's experience of the small pieces of cereal
always being on the bottom.  This is because only the small particles
are able to fall into the small gaps which open up when the box is
shaken, very much like a sifter.

Pub mix after a light shake: the trend from small things at the bottom to big things at the top is pretty clear.

Next, I asked them if we could figure out a way to separate the fine
sand into even finer particles.  We tried a coffee filter, but the
holes in the coffee filter were too small to let any sand through.
Here I made the connection to the atomic theory of matter: water does
go through the holes in the coffee filter, and so must be made up of
very small particles, too small to see.  The same with air; air is able to push things because it is made up of small particles, even though we can't see them.

Finally, we talked about solids vs liquids.  I can pour sand from a
cup, so is it a liquid?  Most didn't want to say it's a liquid but
couldn't say why.  Again, it's useful to point out the progression of
sizes.  The bigger stones could be poured out of a cup but look
nothing like the flow of a liquid.  The finest sand flows more like a
liquid, but not quite.  The liquid has invisibly small particles, so
flows perfectly smoothly as far as we can see.  You can pour sand and
make a pile, but you cannot pour water and make a pile of water!

At the start of each group, I promised them that we would experiment
with quicksand if they made good choices during the main experiment.
The night before, I whipped up a batch of water-soaked sand, which,
with some imagination, could be quicksand. (Quicksand is water-soaked
sand, but apparently not quite the kind of sand we have in our
sandbox!)  This mixture of a liquid plus small solid particles has
interesting properties which are between those of a solid and those of
a liquid.  They had fun with this, but I plan to someday make better
quicksand, perhaps with corn starch.

All in all, I think this 20-minute activity worked very well for the
4-6 year-olds, and I think it will be something they will continue to
experiment with even after my visit.  I limited it to 20 minutes
because we had to get four groups through, but a longer time would be
fine too because many kids wanted to do more sifting.