Riddle

[marycombs]

There is a man that lives on the top floor of a very tall building. Everyday he gets the elevator down to the ground floor to leave the building to go to work. Upon returning from work though, he can only travel half way up in the lift and has to walk the rest of the way unless it's raining! WHY?
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 that fit the initial conditions, only the canonical answer is truly satisfying.