Do as Professor Dan Johnson does and pour about a quarter cup of water into a gallon or 5-liter metal can with a screw top. Place the can open on a stove, and heat it until the water boils and steam comes out of the opening. Quickly remove the can and screw the cap on tightly. Allow the can to stand. Steam inside condenses, which can be hastened by cooling the can with a dousing of cold water. What happens to the vapor pressure inside? (Don€™t do this with a can you expect to use again.)