A stopwatch is a tool used to measure the amount of time something takes. For example, to learn how soon a car can reach 60 miles per hour, or how much time it takes a runner to run 40 yards. These days, stopwatches are even installed in digital watches. You press a button to start the time and press it again to stop the time. A stopwatch can tell you the time in seconds, hundredths of a second, or thousandths of a second.