A man takes the subway everyday to work. He works in the highest level of a highrise office building in downtown. When he arrives he takes the elevator. However, how he uses the elevator depends on if there are other people in there with him. If there are other people, then he'll take the elevator straight up to the top, but if he is alone in the elevator he will take it up only to the 22nd floor, and use the stairs the rest of the way up, about another 20 floors. Why does he do this?

Because he is short, he can only reach the 22nd bottun in the elevator.