This is another lie detector plot. I have another that I also perform which you can read about at https://robertjwallace.com/lie-detector/. This one is easier.
Start by having a card selected and controlled to the bottom of the deck. The convincing control is a good method which you can learn here: https://www.youtube.com/watch?v=78aZh6eUneI&pp=ygUSY29udmluY2luZyBjb250cm9s
False shuffle and/or cut keeping the card on the bottom.
Set the deck down and ask the spectator to cut around two thirds of the cards to the side, to eliminate them. This cut is key. The bottom part of the deck must contain at least 15 cards, but not more than 23.
Pick up the remaining cards and explain that you are going to use the deck to determine the card by asking the spectator to name their card, but allowing the spectator to either lie and name a different card, or to tell the truth and name their card.
Demonstrate by dealing out the cards, spelling the phrase “your card is the”, dealing one card for each letter face down in a pile. Drop the remaining cards on top of those and ask the spectator what their card was. They can lie or tell the truth. Pick up the pile of cards and spell the name of the card they named, one card per letter.
For example if they say the “eight of clubs” you would deal out “e”, “i”…”s” with one card per letter down to a pile, then drop the remaining cards on top.
Now pick up the pile and deal the cards in the same manner, spelling out the phrase “did you lie or not” (14 letters). Turn over the last card dealt (the “t”), and it will be their card, showing whether or not they lied.
So why does the pile need to be between 15 and n cards? Well this is because of the number of letters in the names of the cards. Cards can be spelled out with from 10 to 15 letters. So to spell out the longest card name without running out of cards you need at least 15 cards. Now after the first deal of “your card is the”, the position of the chosen card is equal to the number of cards in the packet minus the number of letters in “your card is the”. The number of letters is 13. That means the chosen card will be at the 2nd from the top if the packet has 15 cards, for example. For the trick to work, that position must be within the number of letters in the name of the card that the spectator names. This is so that the chosen card is dealt by on the second deal. Since the spectator’s smallest name contains only 10 letters, the chosen card must be within the top ten cards after the first deal. The largest number of cards for that to happen is 23 (23 – 13 = 10).
To visualize this algebraically:
n = number of cards in the packet
c = the location of the chosen card
At the start with c at the bottom of the packet, c = n.
After the first deal of “your card is the”, c = n – 13.
After the second deal, c = n – c + 1. Since, from the last step, c = n – 13, this is the same as c = n – ( n – 13) + 1. This will always equal 14 for n = 15 to 23.
Then after the third deal of “did you lie or not”, c with be the last card dealt.
