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gear
Bois du Cazier, the ruins of an old coal mining operation in Marcinelle, Charleroi, Belgium. Credit: Wikimedian Lviatour Luc Viatour.

gear:

A wheel with teeth along its edge. A gear’s teeth fit into the teeth of another gear. As one gear turns, the other gear turns in another direction. If the gears are different sizes, they will turn at different speeds. A set of gears can be used to change speed, power, or direction. A metal rod called an axle is connected to the center of each gear. The axle can be used to drive the wheels of a car or other vehicle. [Gear probably comes from an Old English word meaning “to make something ready for use.”]