Water mills are structures that use the energy of flowing water to perform work, typically grinding grain into flour. They consist of a water wheel that turns when water flows over it, converting the kinetic energy of the water into mechanical energy. This energy can then be used to power various machines. Historically, water mills played a crucial role in agriculture and industry, especially during the Middle Ages. They allowed for more efficient food production and contributed to the growth of towns and trade. Today, some water mills are preserved as historical sites, while others are still in operation, promoting sustainable energy practices.