on the "break a leg" tradition. My understanding is that it is in reference to the curtain call. The curtains on each side of the stage are called "legs." So, during the curtain call, you want to have made such an impression that the audience goes wild and continues to cheer and applaud so long that the "legs" are "broken" open again for an extra bow. if you are really good, you can break a leg several times.
i've also heard that it's in reference to an actual physical injury. this part of the superstition believes that if you wish something good for an actor then they will be jinxed for the show and something bad will happen. with this theory, actors say "break a leg" in order not to jinx them. to me, that doesn't make much sense, so I prefer the above argument.
...and the moral of the story is, don't buy a weedwhacker when your lawn is dead!!!"