This is Awesome

Okay, the science nerd in me is coming out here. I wish I knew how to
do this growing up, it would have been an awesome science project. I
just need to teach Jr. High science, that would be awesome.

Ever looked up in the sky and wondered just how clouds are made? Bill
Nye explains the science of clouds and how you can make one at home.

Get a clear plastic water bottle or soda bottle. Most
sizes--half-liter, one-liter, or two-liter--will work fine. Put just a
few drops of water inside. If you started with a full water bottle, the
water drops left over as you finish drinking will do the trick. Have the
bottle cap ready; set it on a nearby table, for example. Light a couple
matches; the kind from a regular matchbook will work. Blow the matches
out. While they're still smoking, drop the matches into the bottle.
Immediately screw the cap on tightly.

Now for your cloud: Squeeze the bottle, right in the middle. You can
squeeze pretty hard; then let go. A thin cloud will form inside. When
you squeeze it again, the cloud will disappear. Let go again, and the
cloud will re-form. It will work for several cycles. After a while,
though, the bottle leaks, and the smoke particles stick to the inner
wall. The cloud will no longer form. But while it's working, the effect
is quite striking.

How this happens: The droplets of liquid water in the plastic bottle
ensure that the bottle's enclosed atmosphere carries water vapor, as
well. The smoke from the matches provides thousands of particles for
nucleation. In order for the water vapor to become a liquid, it has to
be cooled. Molecules at any temperature above absolute zero are always
in motion, jostling into one another. When you squeeze the bottle, you
compress the air inside; the energy of the moving molecules is squeezed
into a smaller space, and they warm ever so slightly. When you let go,
they cool. And in this demonstration, they usually cool enough to change
to a liquid.

You might wonder why the cloud doesn't form the moment the match smoke
is introduced. It almost does, but the atmosphere in the bottle is too
warm--by just a little bit--to allow this to happen. That slight, slight
cooling does the trick. And you may notice that the trick doesn't last.
After you form the cloud the first time, the heat of the molecules
spreads out again from the smoke particles to the droplets to the
surrounding air, and the liquid water goes back "into solution," as it
is said. The cloud goes away.

Isn't that cool!