A dwarf lives in an apartment building thirty stories high and his room is on the 30th floor. Every day the dwarf goes for a walk around his block, then goes back to his room. On sunny days the dwarf rides the elevator up 15 floors and takes the stairs the rest of the way. (the stairs are indoors) and on rainy days the dwarf rides the elevator all the way to the 30th floor. Why is this?

The correct answer is logical and reasonable.
it also has a 'provable' proof...