A man lives on the 20th floor of his apartment building and every morning he gets in the lift, presses the button for the ground floor and goes to work.
In the evening, he gets in the lift and gets off at the 10th floor and walks to his 20th floor apartment. If however someone who lived on the 14th floor was already in the lift, he would alight the lift and walk from the 14th floor to his apartment.
Why does he do this?
Feel free to ask questions and hide your answer if you know it