Cold water enters the water heater through an internal “dip tube” which forces the cold water to the bottom of the tank and pushes hot water out through the outlet connection. This arrangement provides the most hot water for a given tank size. There are two thermostats on a typical residential water heater. The thermostat at the bottom controls the lower heating element while the top thermostat controls the upper heating element. Usually, only one heating element is active at a time. Because the lower element sees the cold water first, the lower heating element does most of the work, while the upper heating element is used only when the entire tank is cold. Because water can corrode the steel tank, they glass line the inside of the tank and install an anode rod to help reduce corrosion. The temperature and pressure relief valve is a safety device designed to open if the internal temperature or pressure of the tank exceeds certain limits. Water heaters should be large enough to provide adequate hot water even when temperatures are cold and household demand is greatest.