I made up a magic trick today, it's a good one Very hard to spot the numerical property that makes it work if you're being presented to, though typing it out here will probably mean you will spot it instantly
basically, in it's not-spiced-up level it goes something like this:
* roll two dice
* find the sum of the two numbers
* find the difference of the two numbers
* add together those two values (the sum and the difference of the numbers)
* take this number and multiply it by the lower number squared
* say the result
from the result alone, I can determine the values of both dice! How?