Here's an age old "math" card trick...
Take 21 cards from a deck. Have someone select a card from the 21 you split out. Just have them remember the number, not pull the card out.
Start placing the cards into three stacks. One card to each stack 7 times... left to right or right to left... just not both. You'll end up with 3 stacks of 7 cards.
Pick up each stack and show it to the person (becareful to keep them in the same order they were placed into the stack) and ask if the card they picked is in that group. If it is, place that stack of 7 cards between the other two and repeat the sorting process.
Pick up each stack and ask the person if their card is in that group. If it is, place it between the other two and repeat one more time.
The third time, you still pick up each stack and ask if their card is in it... if it is, it will always be the middle card of that group.
AKDejaVu