Variables are also common in science, where they usually represent physical properties that do not stay the same. For example, if a person stands 5 feet away from a building, the "distance" variable compared to the building does not change over time. 5 feet is called a "fixed amount", which is the opposite of a variable amount. But if a person walks away from the building, the distance variable compared to the building is increasing. So it does not have the same value and we cannot use the same number for it, as the distance might be 1 foot now, but 2 feet a second later.