How did water mills work?

Water mills were – and in rare circumstances still are –facilities in which moving water was used as the driving force to power a milling apparatus. The key component in a water mill was a waterwheel, although in later times turbines were also employed, which converted the kinetic and potential energy of water into rotational mechanical energy to drive various machines. Most commonly the mill would grind grain to produce flour. Historically there have been different types of water mill, each largely determined by the type of wheel they used. The kind of wheel chosen was dictated by the local geography and the source of water, with rivers on Mat plains requiring an undershot waterwheel, while those dropping from elevated positions allowing for more efficient designs, like overshot wheels. Today, water mills are used far less due to the availability of more efficient energy-production systems; however, some mills still operate for demonstrative purposes.