Once the water boils, steam will be visible. Remove the can from the hot plate and quickly put the cap on the can. Tighten the cap. Stand back and watch. Background Can Crusher demonstrates an important principle of air, that is, air exerts pressure.
Most people are surprised to see the air exert enough pressure to crush a can.
The thermal expansion of the heated gas molecules inside the can forces some of the air out of the can. After the can is capped and removed from the heat, the gas molecules begin to cool and contract. This leaves areas of very low air pressure. Meanwhile, the air outside of the can is at higher pressure. As soon as it can, the outside air rushes to exert its pressure on the lower pressure inside.