Well, if you try hard enough, you could make a cracking sound when your wrist breaks.
Hmm... Here's a very interesting question I've found: (Not sure if it's imponderable, though.)

"The last man on earth sat alone in his bedroom. Suddenly, there was a knock at the door!"
Can you change one word in the first sentence to make the knock on the door completely impossible?"

Seriously, my dad couldn't figure this one out at all, but I found it in a book called "QuickThinks". (It took me about ten seconds to figure out.)