Conventional water heaters include a tall cylinder, the tank. The tank holds water and a heating mechanism. Water heaters are quite simple, but what they do and how they do it is amazing. Electric water heaters use an immersed heating element. Gas water heaters employ a burner and exhaust system. The tank contains a steel core anode rod made from magnesium or aluminum. The purpose of the sacrificial anode is to retard premature corrosion.