Here's one for you.
Write on a piece of paper the number 1089, fold it up so they don't see it, and hand it to someone.
Have them choose a three digit number. Each digit needs to be different.
Subtract the smaller number from the larger number.
863 - 368 = 495
Reverse that number. 594.
Add 495 + 594
and you get 1089.
The only exception to this rule is if the first and last numbers are consecutive, such as 637 or 928.
If this is the case, the answer will be 198.
736-637 = 99
99 + 99 = 198.
In which case you can put your hand over your brow, and say, "Oh, I'm getting a feeling..." and whip out a second folded piece of paper with 198 on it. :)