A gear is a mechanical device that transfer rotational motion and power from one part of a machine to another. Gears are produced in a wide range of size,and they vary greatly in use. they range from tiny gears that drive the hand of a watch to the huge gears that turn the propeller of a supertanker. A simple gear consist of metal wheel or disc with slots called teeth around the edge. gears alway work in paris. the teeth of one gear mesh with the teeth of the other gear. Each gear has a metal axle in the middle. The axle of one gear is connected to a power source,such as an electric motor. When the power axle turns,its gear turns and cause the second gear to rotate in the opposite direction. This action powers the axle of the second gear to do useful work