Boil water inside the drum until the pressure it exerts equals atmospheric pressure. The vapor inside will push most of the air out of it. When cooled quickly, the drop in vapor pressure produces enough inward force to crush it
You can just heat the can itself up, doesn't need to have water in it, cause the air molecules inside still get heated up, and it's when they cool that the can does the thing, learneded from bill nye
it's because it is heated up and the cold water makes it shrink