The production of a cracking sound by a bull whip is discussed. Photographic evidence shows that the crack is produced by the tip of the whip exceeding the speed of sound and giving off shock waves rather than by the whip slapping itself. This evidence includes motion pictures as well as still shadow pictures which show the tip shedding shock waves. There follows a mathematical discussion of an idealized bull whip with a view to the problem of how the free end of this mechanism achieves a high velocity while the end held in the hand moves relatively slowly. A mathematical solution is given in which it is assumed that a discontinuity in tension propagates down the whip.