The Collapsing Can is one of the classic demonstrations of astmospheric pressure. There are many ways to carry out the demonstration, the simplest of which is just to connect the can directly to a vacuum pump and withdraw all of the air. The vacuum pump method has the advantage that it is fairly easy to follow and hence ideal for modreate and weaker ability pupils.
The demonstration presented here is a little more difficult for pupils to understand as it is not immediately obvious how a vacuum is formed within the can. Nevertheless, the demonstration has a higher impact than the vacuum pump demonstration as it takes a little longer and the collapse is a little more spontaneous.
The key idea is to put a little water in the bottom of the can and then to heat the can until the water is boiling vigorously. The steam that is created displaces the air from the can at which point the can is sealed. When the can cools, the steam condenses and, in doing so, leaves a partial vacuum behind. I have speeded up the process a little by placing the can in a tray of shallow cold water (you can also place the can under a tap of running cold water, but this makes things a little too fast for my liking). Ideally, what I want to achieve is a sense of suspense within my class, followed by a little shock when the can finally goes.