A stopwatch is a handheld timer designed to measure exactly how long something takes, down to fractions of a second. Unlike a regular clock that tells you what time it is, a stopwatch tells you how much time has elapsed since you pressed the start button.

Stopwatches are essential tools in sports. Track coaches use them to time runners in races, swimming coaches measure lap times, and referees time quarters or periods in games. A good stopwatch can measure time precisely to hundredths of a second, which matters when races are won by tiny margins.

Most stopwatches also have a lap or split function, letting you record intermediate times without stopping the overall count. This helps a coach see if a runner's second lap was faster than their first.

Today, smartphones have stopwatch apps built in, but serious athletes and coaches often prefer traditional stopwatches because they're simpler to use while holding a clipboard or standing in the rain. Scientists also use precise stopwatches in experiments where timing matters, like measuring how fast a chemical reaction occurs or how long it takes an object to fall a certain distance.