A clock is a device that measures and displays the time. Most clocks have a face with twelve numbers arranged in a circle and two or three hands that rotate to show hours, minutes, and sometimes seconds. Digital clocks display the time using numbers that change as each minute passes, like 3:45 or 10:22.

People have invented increasingly accurate ways to measure time throughout history. Ancient sundials tracked time using shadows cast by the sun. Water clocks measured time by how much water dripped from one container to another. Mechanical clocks with gears and springs appeared in medieval Europe. Today's atomic clocks are so precise they won't gain or lose a second for millions of years.

The word clock can also mean to measure how fast something happens. Runners get clocked when someone times their race with a stopwatch. A baseball pitcher's fastball might get clocked at 95 miles per hour. When you're watching the clock, you're paying attention to the time, often because you're waiting for something to end (like the last few minutes of school before lunch). When you clock in and clock out at a job, you're recording when you start and finish work.