A man who lived on the top floor of a twenty story building had to go up and down daily for work, and of course, for food and the other necessities. On most days he could only ride the elevator to the fifteenth story, and he would have to walk the rest of the way. When it rained, however, he could ride all the way up to the twentieth story. Why?

“He was afraid of heights.”

This guess is incorrect because the riddle does not suggest that fear is the reason for his behavior. The man's limitation is physical, not psychological.

“He was carrying heavy groceries that made it hard to use the elevator.”

While this could explain why he might choose to take the stairs, it does not address why he cannot reach the higher button when not carrying groceries. The riddle specifies his height limitation as the main issue.

“The elevator was broken on the days he could only go to the 15th floor.”

This guess is incorrect as it implies a mechanical issue with the elevator rather than a personal limitation. The riddle clearly states that he can only reach the 15th floor due to his lack of height.