Ok, I'll start with an easy one that I am sure someone has heard before.
A gentlemen goes into work very early every morning works long hours and goes home with the afternoon rush hour. His office is on the 80th floor of a very tall office building downtown. Every morning without fail he takes the elevator up to the 40th floor gets out and walks up the stairs the rest of the way to his 80th floor office. Every afternoon he takes the elevator from his office all the way down to the lobby and goes home. Why?