A man lives on the 20th floor of an apartment building. If it is a rainy day, he gets into the elevator in the morning, goes down to the ground level 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 does he do this ?
