you could use the one i sometimes use which entails this:
Ask someone to think of a number between 1 and 10 lets say.
Then multiply it by 9.
The result will always be a factor of nine (never!!!)
So then say add the two digits together.
This will ALWAYS add up to 9 e.g 72,81,36 etc.
Then you can do what you like because you know the number is nine.
I.e take away 8 to give 1
Hope that helps, sorry its a bit mathematical and not too magical