A man lives on the 20th floor of a block of flats. If it is a rainy day, he gets into the elevator in the morning, goes down to the ground floor and goes off to work. In the afternoon when he comes home, he gets into the elevator and goes straight to the 20th floor. However, if it is a sunny day he goes down to the ground floor in the morning, but when he comes home he only goes up to the 10th floor and then walks up 10 flights of stairs. Why?
The man is short, and can only reach the elevator button 20 when he has his umbrella.