The Demonstrations: An Aluminum can is crushed using only air pressure!
Quick Physics: When the can is heated, the water inside boils and escapes. When the can is put in the cold water, a partial vacuum is created, crushing the can.
The collapsing can demonstrates that things contract, or get smaller, when they get cold. It also shows that the atmosphere exerts pressure. That is, the air around us pushes on things because air has weight. At sea level, the pressure of air is about 15 pounds per square inch (psi). That is about 150 pounds of pressure pushing on your hand alone! You don't feel it since the pressure is the same everywhere.
An aluminum can is filled with a bit of ordinary water. The can is then heats over a flame or hot plate. As you know, when water is heats it boils and turns into steam. Steam, like air, is a gas, whereas water is a liquid. Gases take up more space than liquids, so we say they are less dense. When the water boils, the can fills with steam and the air is pushed out.
Once the can is full of steam, it is turned upside down into a pan of cold water. This is down for two reasons. First, water seals the opening of the can making it airtight. No more air can get out. Second, the water is cold so it cools the steam in the can very quickly. The cooled steam condenses back to water. But water takes up less space than steam. Since no air can get in to take up the extra space, something else has to give: the can gets smaller.
Really what happens is that the water takes less space than the steam when the it condenses. This makes the pressure inside the can drop. Now the air pressure outside the can is much higher than the pressure inside the can. In fact, if all the air inside the can is gone, that's about 700 pounds pushing on the sides of the can! No wonder the can gets crushed.