For this piece of magic you need a pretty clever audiences who is good at taking away (or can use a calculator).

Ask them to work this sum out:

1. Write down a three digit number where all the digits are different.
   eg 317
2. Reverse the digits eg 713
3. Take the smaller number from the larger number
   713
   317
   -----  -
   396
4. Ask them to tell you the hundreds digit or the units digit and you will be able to tell them the whole answer without knowing what numbers they chose!

THE SECRET:

The answer always has a 9 in the tens column. The hundreds digit and units digit always adds up to 9.

So, if the clever spectators says the hundreds digit is 3 you can work out that the units must be 9-3=6.

So the number must be 396.

If the clever parent says the hundreds digit is 5 that means the units will be 9-5=4 and the whole number will be 594 etc etc

Try it out for yourself, but make sure your spectators gets the subtraction correct!