A fun science experiment is to put a small amount of water in the
bottom of a 1 gallon rectangular can. Boil it for a few minutes to
displace the air inside, remove it from the heat, and immediately
replace the cap and watch.

For more excitement, repeat with another can only this time cap it and
sprinkle cold water on the outside of the can too.

(Hint - don't plan on using the cans again.....!)