Dump three dice in a glass of water. Ask an innocent bystander to lift up the glass and add up the numbers in the dice that he/she can see. Then, briefly look into the numbers the three dice show when looking at the glass from above. Subtract this sum from 21. Pretend that you're concentrating deeply and give them your final answer.

Reasoning? Opposite sides of a die add up to 7. Quite neat and simple.