If you have seen David Copperfield's live show, you might have been wondering about how he did that trick with the Moon Card. Each member of his audience received 9 cards before the show. 8 of them are blank, but one of them has a picture of the moon on it. After a lot of carefully instructed shuffling, David manages to pinpoint the Moon Card in everybody's hand.
Like many of David Copperfield's tricks, the Moon Card trick is based on an old standard that many magicians before him have used. It is commonly known as the 21 card trick, because usually it is done with 21 cards, instead of 9. Magicmeister is a nice online version.
David's Moon Card is just a simplified version of this trick, because he's only using 9 cards instead of 21, and he doesn't let someone chose a card - right from the start he tells everyone that the chosen card is the Moon Card.
You can do the 21 card trick at home: Tell your audience you can read anyone's mind... But it's only a very weak telepathic link, and you can only receive mental images, not complex ideas. The best way to prove it is with a deck of cards (because there are all those pretty pictures on them.)
Ask for a volunteer. Now tell him you have a bunch of cards (no need to mention it's exactly 21 cards) and you will sort them in 3 piles. Ask him to pick a card, but don't let him tell you which card it is. You will just ask him which pile his card is in.
Hold the cards face up in your hand, and take the first card on top, put in on the table. This will be pile 1. Now grab the next card and lay it down next to the first card. This will be pile 2. Grab the next card and put it next to the other two. This will be pile 3.
Now take the fourth card, and place it on pile 1. Grab the fifth card and place it on pile 2. Grab the 6th card and place it on pile 3. In other words, lay down all the cards in 3 piles, by placing one card in each pile and then starting over at pile 1.
After you are done, ask your volunteer which pile his card is in. Then pick up all 3 piles, but make sure that the pile with the chosen card is between the other two piles. Make sure no one notices that you are careful to place the pile with the card in the middle of the other two when you pick up the piles.
Tell your volunteer you are not exactly sure yet - he has to concentrate harder on visualizing the card in his mind so that you can get a clear picture. Tell him to help him concentrate on nothing but the card, you'll make the piles again, and he'll have to look for his card again. Tell him it makes it easier for you to tune into his mental signal.
Now start over - Hold all the cards face up in your hand, grab the one on top, put in on pile 1, put the next card on pile 2, the 3rd card on pile 3, the fourth card on pile 1, etc.
When you are done, ask him which pile his card is in now. (The card may have moved to another pile or still be in the same pile.)
Pick up all 3 piles again. And again, make sure to put the pile with the card in it between the other two piles.
Tell him you are starting to see a blurry picture, and you'll have to make the piles one more time to be sure. Now, for the 3rd and final time... Grab the top card and put it on pile 1, put the second card on pile 2, the third card on pile 3, the fourth card on pile one, etc. etc.
Ask your audience member one final time to tell you which pile his card is in. Then pick up all 3 piles - and again: be sure to place the pile with the chosen card in the middle of the other two piles as you pick them up.
Now act all mysterious as if you were trying to visualize the card. Take each card, hold it for a second, then drop it. When you reach the 11th card, hold it up and show it to the audience - it's the chosen card!
The trick is based on a mathematical principle. By making 3 piles 3 times and always keeping the pile with the card in the middle, you automatically place it in the exact center of the 21 cards... There will be 10 cards before it and 10 cards after it. The chosen card will always be the 11th card.
In David Copperfield's case it's the 5th card, because he did the trick with only 9 cards. So after shuffling, there are 4 blank cards before the Moon Card, and 4 blank cards after it.
PS: You can baffle them even more... Do the trick again, but when you secretly count the cards at the end, do it while the deck is FACE DOWN in your hand. All you see is the BACK of each card, and you will STILL know which card was picked. (Because all you have to do is count to 11, no matter whether the cards are face up or face down.)