A man lives on the 12th floor of an apartment building. Every day he wakes up and checks the weather before he goes to work. He rides the elevator down to the lobby and off he goes.
When he comes home in the evening, if it's a rainy day or if anyone else is in the elevator, he rides up to the 12th floor. But if it's a sunny day with no one else in the elevator, he rides up to the 7th floor and then walks up 5 flights of stairs to his apartment. Why?