Process of Deduction
This is a 2 deck card trick and requires some practice in order to remember the sequence of dealing, selecting and discarding. Is well worth it for the effect.
Take 2 ordinary packs of cards and set them up in advance so that they each have exactly the same cards on the top and bottom. Be sure to memorise them.
Ask a spectator to choose one of the two decks and put the unselected one to the side. Having memorised the cards you w
ll know what the selected deck has as its top and bottom card.
Take the deck which you had put to one side and ask another spectator to cut it into 2 piles. Be sure to note which pile is from the top and which is from the bottom. Ask them to choose one of the 2 piles and put the other to one side. Then ask them to cut the selected pile into two - this time however it's your turn to choose which pile to put aside. Important - be sure that it does not contain the card you memorised.
Hand the remaining pile to the same spectator and ask them to deal it into 4 piles. Before they deal them be sure that you know if the card if your memorised card is on the top or the bottom of the pile. This way you'll be able to follow which of the 4 piles it will end up on. Ask the spectator to select any 2 piles so that you can choose one to get rid of and then put aside the pile that doesn't contain your memorised card. Then point to the 2 piles that you know don't contain the card and ask the spectator which one they want to get rid of. Take that one away. and then put the last 2 piles on top of each other. Important - you need to be sure that your memorised card ends up on either the top or the bottom so you have to put the piles together in the right order.
Now give the stack of cards back to the spectator and ask them to deal them out into 3 piles this time. Point to the piles which you know don't contain the memorised card and then ask the spectator to choose one to get rid of. Restack the remaining cards.
Note: at this point you finish the trick in one of two ways depending upon if there's an even or odd number of cards left.
a) if there's an even number of cards, get the spectator to deal them out into pairs. Then continue the elimination until you have just one pair of cards left. You choose the last card to be eliminated being sure it's not your memorised card which you then turn over .
b) if there's an odd number of cards left and your memorised card is the bottom card, deal them out into two piles and finish by turning over the top card. If it's odd and is indeed your memorised card, ask the spectator to deal into pairs. Then put the bottom card to one side and continue as for the even number of cards above in a).
When you've turned over your memorised card, you then bring back the other deck which you put to one side at the beginning of the trick. Being sure to remember which card was on top and which on the bottom, you turn whichever one matches. Both cards are identical!