Expanding and contracting air has myriad applications; in our home lab we'll use it to inflate and deflate a balloon. Adding heat energy to air causes it to expand and displace cooler air, thus decreasing its density and causing it to rise. It follows that cooling the air has the opposite effect.
Plastic soda bottle
Fill the soda bottle with very hot tap water and let it sit for around 5 minutes to warm it up; empty the hot water.
Put the opening of the balloon over the opening of the bottle. Run the bottle, cautiously, under the hot tap water. You will see the balloon inflate a little.
Now run cold tap water over the bottle. The balloon will not only deflate, but may also be pulled into the bottle quite a bit. Run the hot water again to re-inflate the balloon.
Here are three videos demonstrating what happens:
Sometimes this is done by putting burning paper into the bottle, and instead of a balloon you use a hard boiled egg on top of a jar with just the right sized opening, sucking the egg into the jar. But this can mislead some to think that the flame is creating a vacuum by consuming the oxygen (which is replaced by other gases so no vacuum is formed.) And this version can be reversed in both directions repeatedly, making it a hit if you have multiple children that want to try it out.
Here's a PBS interactive demonstration of what is going on.