Windmills take advantage of the continuous movement of air across the earth's surface. Windmills were originally used for grinding grain and pumping water, particularly where water wheels were impractical. Wind turbines are increasingly used for generating electricity in a manner that produces no air pollution and minimal impact on the environment. Whether powered by water or wind, these types of mills rely on a steady and dependable movement of the driving "fluid." Their particular designs were closely fitted to the local conditions, and display myriad configurations and adaptations. Windmills have sails with geometries that fit the prevailing winds' characteristics, articulations that allow the cap to rotate and face the wind, a tower for placing the sails at the most efficient height, and mechanical drives, gears, and other power train components to use the rotational motion. They show, perhaps even more than water mills, a host of geographical, cultural, and other influences, making them a technology that expresses eloquently much about the society and its history.