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.