"Short Selling", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License
"I'm selling all my analogies at auction tomorrow, and that witch over there will give you 20 beans if you promise on pain of death to win them for her." "What if SEVERAL people promised witches they'd win, creating some kind of a ... squeeze? Gosh, you could make a lot of–" "Don't be silly! That probably never happens."