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Put about two tablespoons of water in an empty aluminum can. Set the can on a stove burner or over a flame until the the water in the can turns to steam. Use an oven mitt or tongs to carefully remove the steaming can from the heat source and immediately turn the can upside down into a bowl of cold water. The whole can does not need to be submerged, just the lip around the open end will do. The can will loudly implode.
If one is available, use a can that has already has the word "Crush" on it.
This demonstrates phase change and the effect of unbalanced pressure. When the water in the can is heated, it changes to steam. This steam fills the can, replacing most of the air that was inside of it. When the inverted can is placed in cold water, the steam condenses, leaving the can mostly empty, and thus with a very low pressure inside of it. The difference between the low pressure inside and the atmospheric pressure outside exerts an inward force on the walls of the can, causing it to implode.
Since there is a loud sound, this may not be ideal for groups of children that are younger than kindergarten-aged. The can may be hard for an auditorium full of people to see without aide, but they will all hear it. After waiting for the water to steam, be sure that the audience is watching closely so as not to miss the rapid implosion.
Shock Value: 7