Can someone tell me good brain teasers?

Theres a person that lives in a hotel on the 20th floor. Every day he takes the elevator down to the lobby and walks to work. (his job is irrelevant) After work is over, he walks back to the hotel, and rides the elevator up to the 15th floor, then walks up the last 5 floors by the stairs, but on rainy days he takes the elevator up to the 20th floor. Why?