Air. It's all around us. Put it to work with this home science how-to, which demonstrates a cool method for crushing aluminum cans with a little bit of ice water and a whole lot of air pressure. To follow along with this experiment at home, you'll need a soda or paint can, a large bowl of water, some ice, and a stove top.
When the water boils, it pushes all the air out of the can. Quickly cooling the can, in turn, causes all of the water vapor to condense, creating a vacuum inside of the can. Because of this vacuum, the air pressure outside the can is greater than inside and the can collapses.