An old bartender's favourite.  Ask your subject to drop three dice in to a glass of water.  Then ask them to hold up the glass, total the numbers on the bottom faces of the dice and then put the glass back down.  You dip your finger tips in the water, mystically rub them on your forehead and magically tell your subject the total they have.

All you do is subtract the total of the dice's upward faces from 21.  Remember, opposite sides of a die total 7 and 3 x 7 is 21.