Tom lives on the fifteenth floor--the very top floor--of a tall building in a large city. There are stairs connecting each floor of the building. But, of course, most people usually use the elevator to get up and down.

In the elevator, the buttons for the floors are in one row, going from top to bottom. The button for the first floor is at the bottom. Thus, when Tom wants to go down to the ground floor, he presses the lowest button.

When Tom takes the elevator up, he always rides all the way to the fifteenth floor if there are other people on the elevator. But if he is alone, he always presses the button for the tenth floor. He gets off on the tenth floor and walks up five flights of stairs to the fifteenth floor.

Tom would rather ride the elevator all the way to the fifteenth floor. But he never does when he is alone. Why not?

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