A cam is a mechanical part that converts spinning motion into back-and-forth or up-and-down motion. Picture a wheel with a bump on it: as the wheel spins, the bump pushes against something, then releases it, then pushes again. That's how a cam works.

Car engines use cams to open and close valves thousands of times per minute. As the camshaft spins, its cams push the valves open at exactly the right moment to let fuel in and exhaust out. Toy machines and music boxes use tiny cams to create movement, like when a rotating cylinder with bumps plucks metal teeth to play a tune.

The word also appears in webcam (a camera for video calls) and camcorder (a portable video camera). When you hear about a cam follower, that's the part that rides along the cam's surface, getting pushed up and down as the cam rotates.

Engineers have used cams for centuries in everything from water mills to printing presses.