This clever idea from Iain Girdwood allows you to deal cards to the table and have a spectator stop you at any point, yet you always finish with a prime number of cards.

Take any deck. You might want to have the spectator cut it in two and use any half. Or just use the complete deck as he'll probably stop you before you get past the half way point.

Say, "I'll deal some cards and you tell me when to stop. I could deal one at a time, but to speed this up I'll deal in pairs."

As you say the above, you deal one card to begin. From now on you deal pairs onto the pile. But you hold each pair waiting for the spectator to say stop (Fig.1). If he doesn't, you deal the pair, then hold the next pair. On only three occasions, if he calls stop, do you replace the pair you are holding on the deck (4th, 7th and 10th). All other times you drop the pair on the pile. See below.

Deal |
a |
pair |
(3) Prime |

Deal |
a |
pair |
(5) Prime |

Deal |
a |
pair |
(7) Prime |

Deal |
a |
pair |
(9) |

Deal |
a |
pair |
(ll) Prime |

Deal |
a |
pair |
(l3) Prime |

Deal |
a |
pair |
(l5) |

Deal |
a |
pair |
(l7) Prime |

Deal |
a |
pair |
(l9) Prime |

Deal |
a |
pair |
(2l) |

Deal |
a |
pair |
(23) Prime |

So, by dealing up to twenty-five cards you always finish with a prime number on the table.

ELIMINATING THE NON-PRIME STOPPING POINTS

One way to eliminate the non-prime stopping points is to use twenty-eight cards. Now do the above but you can deal the pairs freely, because

if he stops you on the three non-prime numbers, the cards in your hand are prime. So you treat the dealt cards as discards.

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