When you boil water the vapor pressure of the water is equals to the pressure due to the weight of the atmosphere (about 1 bar) which is the pressure at sea level more or less. So inside the can while boiling it, the pressure is close to 1 bar of water pressure. Now, by turning the can up side down, you prevent the steam (at 1 bar pressure) from escaping. Now by pouring cold water on the outside, the pressure of the water that was at 1 bar will now decrease because some of the steam will condense ...hence the can now has an internal pressure a little less than atmospheric pressure (about 1 bar) and so the can will be crushed by the weight of the earths atmosphere .that is, the inequity of internal vs. external pressure.

What happens is if you boil an empty pop can, the air inside gets heated.As a result, to maintain constant pressure a lot of air escapes, since heated air has more pressure than unheated air. When you stick it into cold water suddenly, the pressure inside goes waaaay down, and the air can't get back in since it's inside the water. So as a result, the pressure from the air and water crushes it.

The air pressure is actually pretty high! Its simply because there's even pressure all around that you aren't crushed. But if somehow you eliminate the air pressure inside of yourself immediately, you would be crushed pretty easily too =)

Actually, if you seal the can and wait, it will implode at room temperature. The issue is that water vapor displaces the air in the can and water vapor at 1 atmosphere is about 1400x the volume of water at the same temperature and pressure. Once you've replaced the air in the can with water vapor and sealed it, the vapor interacts with the walls of the can, cooling it until a majority of it re-condenses to water. This effectively leaves a substantial partial vacuum in the can. Since normal air pressure is 14.7 lbs/sq. in and the can has about 30 sq inches of surface even a partial vacuum of 10lbs would create 300lbs of force on the can -- more than enough to crush it. Note -- never evacuate glass jars with this trick glass is elastic so that an implosion can make a lot of dangerous shards, people have been blinded.

One way to measure the vacuum you produce is to use a clear plastic jar and then put it face down in cool tap water. As the vapor condenses, waver is drawn up into the jar by external air pressure. If you lower the jar so that the interior and exterior levels are the same, you can measure the volume difference directly.

Gas occupies an amount of volume proportional to the temperature of the gas. If you chill off a gas quickly, it will contract. However, the pressure outside of the can will remain the same, because it is the pressure of the atmosphere, and so if you suddenly chill the gas inside of it, then the pressure outside will be stronger than the pressure inside, and it will crush the can.

As the temperature increases, the air inside the can will expand. As the air expands, some of the air can now "escape" out of the open top. While everything is hot, there is now less air inside the can than while it was cool. By quickly turning the can over and quickly putting it into cold water, the remaining air inside the can is suddenly cooled. As the gas cools, it loses its kinetic energy which results in a lower pressure inside the can. Since the gas inside the can is at a lower pressure than the outside of the can, the can implodes.

Think about water. What happens when you heat it? What happens to steam when you cool it? What happens to air when it is heated and cooled? You could try putting a balloon filled with air into the freezer. What happens to it? Does that give you ideas about why the coke can with a little boiling water in it collapses when you turn it upside down in cold water? Do you think it would collapse if you put it in cold water without turning it upside down? Why or why not?