A ballcock is a mechanical device used in toilets to control the flow of water into the tank. It consists of a float, usually shaped like a ball, that rises and falls with the water level. When the tank is empty, the float drops, opening a valve to allow water to fill the tank. Once the water reaches a certain level, the float rises and closes the valve, stopping the flow. Ballcocks are essential for the proper functioning of toilets, ensuring they refill after each flush. They are often made of plastic or brass and can be found in various designs, including the traditional toilet ballcock and newer fill valves.