Here's a very basic math exercise that won't amaze your friends but will get them thinking. I like to pretend I am opening an envelope a la Johnny Carson's Carnac or have the answer written on my arm or something.
Pick a three digit number of different numbers (so 123 or 487 are OK but 111 or 232 are not) Let's use 816 as an example.
Reverse the number. So 816 becomes 618
Subtract the smaller number from the larger. 816-618=198
Reverse the answer number. 198 becomes 891
Add that number to the answer of the subtraction. 891 + 198 = 1089
Voila! The result is 1089!
It's simple to prove with a little Algebra - Let the Geek know if you can't figure it out. It's based on the places of the digits so if you choose a number like 546 the process will be 645-546=99 and 99+99=198 not 1089.
You have to look at it like 99 is really 099. So 990-099=891 and 891+198=1089
Note also that it may not work with repeated digits because if it's a palindrome like 575 when you reverse it you will also get 575 and the difference will be 575-575=0 so there is nothing to work with.