There are many ways you can use the magic of math to make it look like you're reading minds. The secret to tricks like this is to have your participant do a series of math problems that will always give him the same answer. For example, have him think of any number, multiply by two, add 10, divide by two and subtract the original number from that total. The answer to this sequence will always be five. However, if you want to impress your audience, it's better if you don't simply give him the answer at that point. As with any magic trick, presentation is key. Have him perform a few more math equations; since you know from that point on that his total was five, you can easily calculate the result of any future equations. Once you have a total, don't just tell your spectator his number; rather have him use the number for something else. For example, you could have him look up that page number in a book you give him. Before performing the trick, you would have looked up that page and memorized the first sentence on your chosen page. When your spectator does the series of equations and comes up with the final number, you give him the book and tell him to look up that page number and read the first sentence silently. You then recite the sentence to him from memory. By drawing the audience's attention away from the math equations and numbers to a seemingly random sentence from a book, you make the trick much more impressive.
Card tricks are one of the easiest types of mind reading tricks. There are many different ways to make it look like you are reading minds with cards. All of these tricks involve your participant choosing a card without telling you; you then have to figure out which card he picked.
One type of card reading trick involves methods of shuffling cards so that you can place the card your participant picked into a particular spot in the deck. For example, pick 25 cards from a regular 52-card deck and lay them out in five rows of five. Place them face up, going from left to right. Ask your participant to choose a card and tell you which horizontal row her card is in. Memorize the card on the far left side of that row. Then pick the cards up, starting at the bottom right corner. Place the card at the bottom right corner on top of the card above it; then place both of those cards on top of the card above, and so on with each row, till you have five stacks of cards in a horizontal line. Then take the stack on the far right and place it on top of the stack beside it and continue stacking the cards from right to left till you have a single stack. Next, start with the card on the top of your stack, with the cards still facing up, and lay them out in five rows of five, placing them from left to right and top to bottom. Again, ask your participant to tell you which row her card is in. Then find the card you memorized from the left-hand side of her earlier row. The card that lies in the same vertical row as the card you memorized earlier is the card your participant picked.
If you are performing this type of trick, you don't necessarily have to tell your participant right away when you know which card she chose. The more shuffling and rearranging of the deck that you do after you have figured out her card, the more impressive your trick will look. For example, you could then pick the cards up and turn them face down, being careful to place her card either on the top or bottom of your pile so you know where it is. Then place the cards face-down in groups of four. Memorize where you have placed your participant's card. Then ask her to choose a group of cards. If she chooses the group with her card in it, discard all the other groups; otherwise, discard the group she chose. Keep doing this till only the group with her card in it is left and then do the same asking her to choose a card from the group. In this way, you can gradually discard all the cards in the deck till the only card left is the one she originally picked.
Another type of mind reading card tricks involves sleight of hand. Sleight of hand requires practice, but it is often relatively simple to perform. One easy example is "Stop When You Like." For this trick, before you start, have your deck face-down and look to see what the bottom card is. Hold the deck in your palm with your thumb on the bottom and your fingers on top. Use your fingers to slide the top card down from the rest of the deck, separating it from the deck. Do this again with the next card and continue with the next, going on down through the deck. Ask your volunteer to tell you to stop when he likes. When your volunteer says stop, pull the separated cards off from the top of the deck and show him the card where he stopped. This is his card. However, instead of showing him the card from the middle of the deck, slide the card from the bottom of the deck off when you pull your palm back with the cards in it. That way, the card you show him will actually be the card you memorized from the bottom of the deck. You can then reveal to him what his card was. You could do this by writing it down before you begin the trick and showing him the paper where you wrote it, or by turning the deck over face up and finding his card.
A final category of mind reading tricks involves using the responses that most people will give to a specific question. The key to these tricks is usually to get your audience to answer quickly, without thinking about their answer. If they spend time thinking about an answer, they will be less likely to give the most common answer. The easiest way to ensure they answer quickly is to ask your question in the middle of a series of other questions that require quick answers.
For example, try the magic vegetable trick. Begin this trick by writing "celery" on one side of a piece of paper and "carrot" on the other side. Put this paper in your pocket. Next, give your participant a piece of paper, and have them "warm up" by answering a series of simple math questions. Don't let them pause to think; just have them write down the answers as quickly as possible. Then have them write down the name of a vegetable. Again, don't let them pause or think about it--tell them they must write the first thing that comes to mind. For most people, the vegetable they think of first will be carrot, and most people who don't think of carrots will think of celery. Once they have written it down, show them the side of the paper where you wrote the same vegetable that they did.
Be careful with these kinds of tricks that rely on the answers most people give; they can always backfire. An unusual participant might give you an answer that is completely different from the one that most people will think of. However, most of the time these kinds of tricks work; they are especially impressive because it's so difficult to figure out how you knew what the answer would be.