Most cans can be crushed by hand, but this metal can CRUSHES ITSELF in front of your eyes! See how you can easily teach science with this fantastic classroom DEMO, but please caution the students NOT to try it at home (it requires heating and handling hot objects, and should only be done with supervision).
Thoroughly wash out an empty gallon-sized metal can with screw-on cap (or you can use a rubber stopper). You need one cup water, a hot plate or stove, and a thick mat or newspaper stack for the hot can, and a large cloth soaked in COLD water.
Pour the cup of water into the can, leaving the opening on top uncovered. Teacher places the can onto heat until it reaches the boiling point, and letting it boil several minutes so that steam replaces most of the air inside the can. Using a pot holder, teacher removes the can from the heat, and sets it onto a heat-proof surface. She immediately screws the cap back on (or puts rubber stopper into opening).
She covers the can with a large wet cloth, and waves her hands over it, saying a few magic words. Suddenly, the class hears some crackling sounds and sees the cloth shrink down. When noises stop, teacher removes the cloth to show a crushed can!
SCIENCE behind the magic:
Boiling water produces steam, which forces air out of the can, and replaces most of the air in the can. The can is closed up before the steam can condense. But, as the steam condenses inside the can, the pressure inside DECREASES… then the outside air pressure is suddenly GREATER than the inside pressure, and that force pushing inward on the can causes it to COLLAPSE onto itself. (A most impressive display, especially using a gallon can, not the usual soda type.) Do it with as much DRAMA as I do on stage (see photo) and you will get even more response from students!