Water is added to an empty soda-pop can and heated to boiling.
Can is then quickly inverted into cold water which causes it to collapse. Or a larger gas can is used and instead of being inverted into the water after heating the water inside, the can is stoppered and allowed to cool in water or in room temperature air. When water vapor inside the can is cooled it condenses, decreasing the pressure on the inside of the can. With the pressure on the outside of the can greater than that on the inside, the can will collapse. When water is boiling and water vapor escapes the can, quickly invert can into pan of cold water.