"Break a leg!" One says this to actors (and other performers) before they go on stage, by way of wishing them success, especially on opening nights. But where does the expression come from and why is it used in this way? There may be some connection with the Dutch phrase "hals en been breken" (break a neck and a leg) and with a similar one in German, but this still doesn't explain anything. I've also heard it mooted that this colorful figure of speech derives from John Wilkes Booth having leapt from the presidential box and broken his leg after assassinating Lincoln. But why? Because he succeeded in his appointed task, at a play?