Tom lives on the fifteenth floor of a tall block of flats in a large city.He usually uses the lift to get home.the button for the first floor is at the bottom,the button for the fifteenth floor is at the top.it there is somebody in the lift,he always presses the button for the tenth floor.he gets out on the tenth floor and walks up to the fifteenth floor.why doesn't he go to the fifteenth floor when there is nobody in the lift?