A whip cracks because when it is "whipped" properly, the end of the whip actually breaks the sound barrier. By swinging the whip back and then jerking it forward in a properly timed motion, the end, which is moving rapidly backward, is forced to very quickly change direction and go forward. It is at this transition that, if the motion of the individual wielding the whip is adequate, the end reverses direction so quickly that it breaks the sound barrier and the crack! of the whip is heard.