How to perform it
- Hand over the dice. Let the volunteer examine and roll them. They are ordinary dice.
- Build the tower. Turn away, and ask them to stack the three dice into a single column in any orientation they like.
- Glance at the top. Turn back and look at the number on the top face of the stack, as briefly as you can.
- Do the sum. Subtract that number from 21. That is the total of the five hidden faces: the bottom of the top die, the top and bottom of the middle die, and the top and bottom of the lowest die.
- Announce it. Write it down or say it, then have them unstack the dice and add the hidden faces themselves.
Why it works: opposite faces on a die add up to 7. The top and bottom of each die make 7, so three dice make 21. The only face you can see that counts toward that 21 is the top one; take it away and you have everything hidden.
Presentation
- Don’t look at the stack too long. The sides of the dice show plenty of numbers, which makes it look like you are calculating from them. Let the audience think so.
- Do it with more dice. With four dice the total is 28 minus the top face; with five, 35 minus the top. Seven per die.
- Check your dice. Almost all standard dice follow the opposite-faces-add-to-7 rule. Novelty dice sometimes do not, so test before you perform.
Credit where due: A classic dice principle. On a standard die, opposite faces always add up to 7: 1 and 6, 2 and 5, 3 and 4.
Next: The mind-reading number cards