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.