This is taken from the Car Talk puzzler of the week, seen here: http://www.cartalk.com/content/mathematic-mistake-0?question
I'll summarize it thusly:
A hotshot mathematician calls a press conference because he's found a counterexample to Fermat's Last Theorem (which claims that $A^x + B^x = C^x$ has no integer solutions for $A, B$ and $C$ when $x > 2$). However, just to be dramatic (and annoying), he doesn't reveal the whole counterexample, but just the values of $A, B$ and $C$, which are 91, 56 and 121, respectively. The 10-year-old child of one of the reporters attending the press conference raises his hand, and says "Sorry, sir, but you're wrong."
The question is: How did the child know?