Dan Stamm, a physics teacher at Campbell High School in Smyrna, GA constructed a platform for heating water in a steel drum. The drums used had a surface area of about 3000 square inches. The force of one atmosphere of pressure on that area is about 44,000 lbs or 22 tons! Nevertheless, such cans can be made of reasonably thin steel because the same force normally acts outward from the inside of the vessel. The procedure involved pouring about a gallon of water into the drum (just enough to cover the bottom) and and heating it over a propane burner for 15 to 30 minutes. The water is brought to the boiling point where its vapor pressure is equal to atmospheric pressure. The vapor will then push most of the air out of the can. The can is then sealed as the heat is removed. If the drum is cooled quickly, the sudden drop in vapor pressure will result in enough net inward pressure to crush the can.