A man lives on the 13th floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets into the elevator, and, if there is someone else in the elevator- or if it was raining that day...he goes back to his floor directly. However, if there is nobody else in the elevator and it hasn't rained, he goes to the tenth floor and walks up three flights of stairs to his room. How come?