Differential calculus
Unlike a number such as 5 or 200, a variable can change its value. For example, distance and time are variables. At an Olympic running race, as the person runs, their distance from the starting line goes up. Meanwhile, a stopwatch or clock measures the time as it goes up. We can measure the average speed of the runner, if we divide the distance they travelled by the time it took. But this does not say what speed the person was running at exactly 1.5 seconds into the race. If we had the distance at 1 second and the distance at 2 seconds, we would still only have an average, although it would probably be more correct than the average for the whole race.