A mathematician, called a genius by some, is working on a new equation that will change the way we think, kind of like E=MC2. It covers 9 blackboards and it's almost done, but he can't figure out the end. Out of frustration he says, "Damn, I'd sell my soul to finish this."
Immediately the Devil appears, wanting the mathematician's soul. Of course, the math dude takes it back, saying it was a figure of speech and he didn't really mean it. The Devil, for whatever reason, gives the math dude a choice. Either he loses he soul or he can ask the Devil 3 questions and then ask of him something he can't do or say.
Being a genius, the math dude takes the second option, figuring he can come up with something the Devil can't do or say. He thinks about his first question for about 1/2 hour, then asks, "Is there anywhere you can't go?". The Devil laughs and says he's been around since the beginning of time and has been everywhere, of course there's nowhere he can't go.
The math dude takes an hour think of his second question, then asks the Devil, "Is there anything you can't say?". The Devil laughs again and says that he's no mere mortal, there is nothing in the world he can't say.
It takes the math dude much longer to think of his third question, then asks, "Is there anything you can't physically do?", to which the Devil replies he can take any shape and can become whatever he chooses, be it animal, human, plant, rock or anything else.
Now the Devil says, "You've asked me three questions, now ask of me something I can't do or say." The math dude thinks for a long time, then comes up with something. The Devil, in disbelief of what the mathematician came up with, gives him back his soul in disgust.
What did the mathematician ask of the Devil that he can't do or say? Remember that the Devil is immortal and can go anywhere, including Heaven, and he can give the math dude his soul back if he (the Devil) so desires. It also has nothing to do with the equation.
EDIT: I'll answer any questions that won't give away the answer
