Solar water heating systems for buildings have two main parts: a solar collector and a storage tank. Typically, a flat-plate collector—a thin, flat, rectangular box with a transparent cover—is mounted on the roof, facing the sun. The sun heats an absorber plate in the collector, which, in turn, heats the fluid running through tubes within the collector. To move the heated fluid between the collector and the storage tank, a system either uses a pump or gravity, as water has a tendency to naturally circulate as it is heated. Systems that use fluids other than water in the collector's tubes usually heat the water by passing it through a coil of tubing in the tank.