What happens when water vapor in a can suddenly condenses? Put a small amount of water in an aluminum softdrink can, and then heat the can on a stove until steam comes out of the opening. With a pair of tongs, invert the can into a pan of cool water. Air is driven from the can so that only water-vapor molecules remain inside. When the can is inverted into the pan of water, the water vapor condenses. The condensation of molecules in the can leaves behind a vacuum. The pressure of the atmosphere, with nothing inside pushing back, crushes the can.