Demonstrating Atmospheric Pressure?

You need a heat source (kitchen stove) an empty aluminum soft drink can, a hot mitt, a small pan filled to a depth of 1/2 inch with room temperature water. Put a spoonful of water into the can. Place the can on the heat source. When it sounds as if boiling is about to take place, quickly take the can from the heat source using the hot mitt and place it upside down into the water in the pan. What happens? Why?