Leonardo da Vinci designed a water mill to use flowing water for grinding grain.
A water mill transfers the energy of moving water to a wheel. The wheel turns an axle and gears, which can drive millstones or other machinery. Leonardo studied water-powered machines extensively because mills were among the most important sources of mechanical power in Renaissance Europe.
His notebooks contain detailed observations of water flow, channels, wheels, gears, and methods for regulating motion. These studies were not limited to flour production: the same principles could operate hammers, saws, pumps, and other industrial equipment.
Leonardo did not invent the water mill. Water-powered mills had been used for centuries before his lifetime, including in the Roman world and medieval Europe. His contribution was careful analysis and imaginative refinement of existing hydraulic technology rather than the creation of the basic machine itself.