The instructions you give
- Pick an hour. “Think of any hour on the clock, one to twelve. Don’t tell me.”
- Explain the count. “I’m going to tap numbers. With each tap, count silently in your head, but start counting from one more than your hour. If you picked 5, the first tap is 6, then 7, and so on.”
- Say stop at twenty. “When you get to twenty, say stop.”
The secret rule
- Taps one to seven: anywhere. Tap any numbers at all for your first seven taps. Wander around the clock. It looks random because it is.
- Tap eight: twelve o’clock. On your eighth tap, tap the 12.
- Then go backwards. From then on, tap 11, 10, 9, 8 and so on, moving one hour counterclockwise per tap.
- Stop when they say stop. Your finger is on their hour. Leave it there and let them look.
Count your own taps silently up to eight. After that you do not need to count at all: just walk backwards around the clock until they say stop.
Why it works: someone thinking of hour h says stop on tap 20 − h. From tap 8 onward you are pointing at hour 20 − (tap number). Put those together and you are pointing at h exactly when they stop. Since h is between 1 and 12, they always stop somewhere from tap 8 to tap 19.
Practise the rhythm
Tap at a steady, slow pace, about one tap a second, so your volunteer can keep up. If they say stop before your eighth tap, they have miscounted; laugh it off and start again with a clearer explanation.
Variations
- Hours on a wall clock. Use a real clock across the room and point instead of tapping. It looks even less like a method.
- Jump about early. Make the first seven taps leap across the clock rather than move in order. The steady backward walk that follows then reads as you homing in on their thought.
Credit where due: A classic self-working effect. We checked the rule for all twelve hours: in every case the spectator's count reaches twenty on the tap where the performer is pointing at their hour.
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