Another trick that works with most graphing calculators (probably scientific calculators too, though I don't have any to test) deals with non-integer factorials. I stumbled onto this after getting bored in math and trying out non-integer factorials. If you do (0.5)!, you get Sqrt(Pi/2). (-0.5)! gives you Sqrt(Pi). It's a pretty bizarre thing to see, especially as the factorial operation is technically only defined for whole numbers. It's especially fun to show to your math teacher (mine actually stopped teaching for the rest of the period to try and work out why this happens).

Thanks to MathWorld and an excess of free time, I found out a few days later that it was because most calculators use the Gamma Function of (n+1) to find the factorial of n rather than the recursive definition one would expect.

I successfully tested this on the following graphing calculators: TI-83, TI-83+, TI-83+ Silver, TI-86, TI-89, HP-49G+.