Why do closed containers sometimes dent or even collapse on cool days?
Since our bodies have air inside them, the air inside our bodies is at the same atmospheric pressure as the air outside our bodies. Therefore, the pressures are equal and we can move quite freely in our atmospheric environment. The remainder of our body is mostly liquid water and cannot be compressed.
The same cannot be said for divers. As the divers go deeper and deeper, the water pressure increases. At a depth of 10 meters (32 feet) it is about twice atmospheric pressure. A diver can breathe in compressed gas to balance this pressure. If a diver goes to extreme depths and then ascends too rapidly the diver can suffer the “bends.”
Just as a balloon can become smaller when placed under water, a sealed container can change its shape and even collapse under certain atmospheric conditions. For example, a container that stores gasoline for a lawnmower is usually sealed when it’s not being used. If the container were sealed on a warm day, when there was little atmospheric pressure, subsequently on a cool, high-pressure day, the gasoline container could be crushed. A second reason is that gasoline vapor exists in the tank, and at low temperatures more of the vapor condenses to liquid, reducing the pressure of the gases in the tank.