This is probably the best known and most celebrated of all lateral thinking puzzles. It is a true classic. Although there are many possible solutions which fit the initial conditions, only the canonical answer is truly satisfying.
Solve: The Man in the Elevator [Lateral Puzzle]
A man lives on the 20th floor of a building. Every morning he takes the elevator down to the lobby and leaves the building. Upon his return, however, he travels halfway up in the lift and walks up the stairs to reach his apartment. He hates walking so why does he do it?
Hint: He is very proud, so refuses to ever ask for help.