One of the surgeons I was working with showed me this one, and it's kind of a fun party trick. Sit in a chair and lift your right foot. Start rotating your foot in circles, Clockwise (to the right). Now, with your right hand, draw the number 6 in the air- your foot will automatically stop going clockwise and start turning counterclockwise! It's the darnedest thing. There is no way for your foot to keep going clockwise. Go ahead, try it. You know you want to.
tough, not impossible like rubbing your belly with one hand and patting your head with another at the same time ( and not somebody else's another )