The Puzzle Challenge: Elevator Habits

Every day, when Grant gets home from work, he takes the elevator from the ground floor to the 16^th floor. He then gets out and takes the stairs up the last four floors to his apartment on the 20^th floor.

Why?

Clues: he only does this if he is alone in the elevator. Otherwise, he takes the elevator all the way to the 20^th floor. When he leaves home, he takes the elevator straight from the 20^th to the ground floor, regardless of whether he is alone in the elevator.

Can you solve the puzzle? Email me back with the answer at [email protected] and I'll post the winners, along with the answers on the blog.