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You said it! The one where the tone of voice gives it all away
The magic effect
You claim you can tell what is on your spectator's mind just by the tone of their voice. To prove it have them deal the cards into six piles. They chose a card from the centre of one pile, then put the piles back together in whatever order they like. They cut the pack. Finally they deal the cards out one at a time, saying their name out load as they do so. When they get to it you can instantly name the card they selected, you claim because their voice gives it away. Brilliant bluff detection or just baloney?
Peoples voices do change a little when they lie, but it's very hard to detect. This is just a trick! First remove the ace of clubs from the pack, so your spectator is actually using a deck of only 51 cards (but they won't notice that, and you won't tell them). Next, set any six of the remaining clubs on the top of the deck and the other six clubs on the bottom. The order isn't important. Now, on with the demonstration of your 'wonderfully tuned ear'.
When they deal the cards into six piles, the bottom card and the top cards of every pile will be a club. That's all the clubs used up (remember no ace). They select a card from the middle of the pile (important to stress this: not from the top or bottom but from the middle). They then place it back on top of any pile and reassemble the deck form the six packets. Because clubs were top and bottom of every pile their chosen card will be sandwiched between two clubs whichever pile they chose. Even if they cut the pack, their chosen card will still be the filling in a club sandwich.
That's your cue. They name the cards as they deal them down one by one. You listen for a club, then a card then a club. That pattern lets you say stop. Name their card (the one between the two clubs) and take the applause.
Prove it works!
There was a club, on top of every pack. The chosen card was placed on top of a pile so is now on top of a club. When the piles are put together, some other pile is placed on top of the chosen card. There was a club on the bottom of every pile so a club will now be on top of the chosen card.
The above doesn't hold only if they decide to make the pile with their chosen card the top pile. Then the chosen card will be on top so not have a club before it. However, the very bottom card of the pack is a club as it was the bottom of one of the piles. When they cut the pack, they will make the club-X-club pattern!
There is, however, a chance that the cut will fall immediately next to the chosen card. In that case there will be no club-X-club pattern. Suppose the cut falls just above the chosen card. The chosen card is now placed on top with a club after it. A club is left as the last card. On the other hand, if the cut is just below the chosen card, then that card will be left as the bottom card with a club just before it. A club will be on the top. In either case, if you did not hear the pattern, then think back to which of these two situations (non-club: club on top versus club: non-club on the bottom) happened and you can still say which the chosen card was.
Could the cut create a second club-X-club pattern? A scientist would call this a false positive. We know there is a club at the bottom of the pack. This could therefore only happen if the cut brought a single non-club card followed by a club to the bottom. However, the card that comes to the bottom is the top card, and it is a club too. Aside from the situation above where the chosen card ends up as the top card, the cut will only make a club-club pattern.
Therefore the trick will always work!
Set this up as a demonstration of skill. Emphasise that this is something that takes years of perfection. You are not playing the role of conjurer but of someone who has developed an extra-ordinary skill through practice, practice, practice.
Demand perfect silence from the audience whilst the card reading part is done. After all, you are supposed to be listening for subtle cues a normal ear would miss. Sound a little unsure when you make the prediction to emphasise how hard it is.
If you want to make it more amazing you can allow members of the audience to do several cuts. That increases the chances of destroying the pattern of course with you having no way to know. If so you can rely on the standard excuse of psychics. Smile ruefully, point out that "It's not an exact science", and move on to the next trick!
The computer science
This trick relies on the audiance not knowing about the set up of the pack. If they realise where the clubs are it is all very obvious what is going on. The stack has what a computer scientist would call hidden state.
A similar thing goes on in computer programs. Think of a typical digital watch for example. Even a simple one has multiple modes. It might have one for showing the current time and another for showing the time the alarm is set for. If there is no way to tell, looking at the watch, which of these two modes it is in, then you have a hidden state. You could accidentally put it into alarm mode, then later glance at it and think that was the time. Just as in the trick you cannot tell what is going on and make bad assumptions about what is happening as a result.
What this means for interface designers, if they are dealing with a system with multiple modes, they need to find ways to make the modes visible in the interface, so the user can tell exactly what is happening. Avoid hidden state except where the user never needs to know it. After all, there is no point worrying about the subtle issues of attention and awareness in your designs, if the information needed isn't there in the first place.