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Mass hypnosis: the one where you force everyone to think of the same card
The magic effect
You explain that all great magicians can make a volunteer pick a card of the magician's choosing. You are going to try something much harder. You are going to use your powers of suggestion to make as many of the audience as possible pick the card you choose, all at once. You show five playing cards on a screen. The audience focus on the first card their eyes settle on and then look at no others. Once they have picked a card they raise their hand. Once all the hands are up, you blank the screen and tell them to keep concentrating – they must remember their card and theirs alone. You explain that you will then deal out the cards again but you will have removed one card. It is the card you think most people will have settled on. Once you show the cards, anyone who chose the card that is gone should put their hand down. You show the cards again and all hands go down down. You made them all pick the right card!
This trick is simple. You just swap allthe cards for new ones. As none of the cards will remain it doesn't matter what cards anyone chose. To be effective you need to use very similar, nondescript cards: 7s, 8s and 9s of similar red and black suits. That ensures the differences in the cards they didn't choose are not very noticeable.
Prove it works!
Trivial. They all change so of course everyone should put their hand down.
To increase the effect you need to give the audience an expectation of what you are about to do. Claim that you are going to try an experiment on the power of suggestion. Play up the point that what you will do requires a deep understanding of psychology and the ability to make subtle suggestions. Point out that as there are five cards, from chance alone, one in five people will sit down. For the experiment to work you will need to make far more people than that choose your card.
Read on to discover the linked computer science