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The air around you is pressing against you and every other object, with a pressure as high as 101 kPa (14.7 pounds per square inch) when you're at sea level. This would normally be enough to crush a can by itself, or even a person! This doesn't happen because the air inside the soda can (or the material inside your body) is pushing outward with equal pressure, and because the air pressure "cancels itself out" by pushing at us from every direction equally. When the water in the can boils, you can see the water start to escape as little droplets in the air, or steam. Some of the air in the can gets pushed out when this happens, to make room for the expanding cloud of water droplets. Despite the can losing some of the air inside it, it doesn't get crushed yet, because the water vapor that took the place of the air is pushing from the inside instead. In general, the more you heat a liquid or a gas, the more it expands. If it is an enclosed container so it can't keep expanding, it exerts more pressure. This is known as Gay-Lussac's Law. When the can is turned upside down in the ice water, the situation changes in two ways. First, the can is no longer open to the air, since water is blocking the opening. Second, the water vapor inside the can rapidly cools down again. The water vapor once again shrinks down to its original volume, the tiny amount of water at the bottom of the can. Suddenly, most of the space inside the can has nothing in it at all not even air! The air that's been pressing from the outside of the can this whole time suddenly has nothing on the other side to resist it, so it crushes the can inward. Space that has nothing in it is called a vacuum. The appearance of a vacuum, or empty space, inside the can has one other effect besides causing the can to be crushed. Watch the can carefully as you lower it into the water, and again as you lift it out. You might notice a small amount of water getting sucked up into the can, then trickling out again. This is because the water pressure is pushing against the opening of the can, but only hard enough to fill a little of the can before the aluminum is crushed.
Learn about air pressure. Figure out what happens when you heat the can of water. Understand how the can gets crushed. Watch the can closely to discover one more effect of the experiment.