How to perform it
- Hand over the coins. Give the volunteer a penny and a dime. Turn away, or ask them to put their hands behind their back, and have them close one coin in each fist.
- Assign the values. Remind them: the penny is worth 1, the dime is worth 10.
- Right hand, times three. “Take the value of the coin in your right hand and multiply it by three. Keep it to yourself.”
- Left hand, times two. “Now the coin in your left hand: multiply its value by two.”
- Add them. “Add those two numbers together and tell me the total.”
- Read the total. If the total is odd, the penny is in their right hand. If it is even, the penny is in their left hand.
The only totals possible are 23 (penny in the right hand: 3 + 20) and 32 (dime in the right hand: 30 + 2). Do not let them notice there are only two possible answers: take your time, and act as though the number told you something subtle.
Why it works: multiplying by two always makes an even number, so the left hand never changes whether the total is odd or even. Only the right hand counts: 1 × 3 is odd, 10 × 3 is even.
Make it look less like arithmetic
- Ask for the total, not the parts. Never ask what each hand came to.
- Repeat with a change. The second time, change the multipliers, for example ×5 on the right and ×4 on the left. Any odd number on the right and even number on the left works the same way.
- Do not use a nickel. A nickel is worth 5, an odd number like the penny, so the method breaks. Use a penny and a dime, or any pair where one coin’s value is odd and the other’s is even.
On a video call
This one works well on camera because the volunteer holds their own coins. Ask them to hold their fists up to the camera at the end, then name the hand.
Next: Heads or tails, unseen