Ask any woodworker if he can drill a square hole, and he'd say that of course ... with a hollow square mortise chisel & bit. But that's kind of cheating: it's basically drilling a round hole and chiseling the rest to make the square hole.

But did you know that there is a way to actually drill a square hole? With a Harry Watts square drill bit (named after its inventor, who patented it back in 1917). Here it is in action:

Confused? Here's a neat animation that shows what exactly is happening (hint: the bit is shaped like a Reuleaux triangle):

Pretty nifty, huh?