A gear's teeth mesh with the teeth of a neighboring gear - transmitting the rotary motion

A gear is a wheel with teeth on its outer edge. Gears rotate on a central axis and work with other gears to transmit turning force. The teeth of one gear mesh with — or engage — the teeth of another gear.

The rotating force produced by an engine, windmill, or other device often needs to be transferred or changed in order to do something useful. For example, as you pedal a bicycle, you cause the sprocket to rotate. But in order to make the bike move, this rotating energy must be transmitted to the rear wheel.

Gears are used to transmit turning force. They can also change the amount of force, speed, and direction of rotation.