Courtesy costs nothing. Lower the toilet seat after use. Let the people in the elevator exit before you enter. Return shopping carts to their designated areas. When you borrow something， return it better shape (filled up， cleaned) than when you got it.
Time is the invisible material in all my works.
计算机是一种交互式媒介，是第一个允许用户操纵过程表示的媒介。