I watched a video explaining a card trick today, there were no tricks, it was all based on math. Here's the set up: 52 cards in the deck. person A takes some of them, then person B takes some as well. The dealer counts the remaining cards in his hand, say he has 18. Then he says he'll make a prediction. he does these calculations in his head: 1 + 8 = 9, and 16-9=7. So he looks through his cards and pulls out a seven, and sets it aside face down. (persons A and B are unaware of what the card is) Then person A counts the cards in his hand, suppose he has taken 19. The dealer says 1+9=10 and 1+0=1, so person A has a 1. Person B counts his cards, and he has 15. So he says then that 1 + 5 = 6, so person B has a 6. Then he says that since person A has a 1, and person B has a 6, that his card is 1+6 = 7, and flips over his 7 to end the trick. I find this trick to be truly fascinating. I have no idea how to begin answering why this trick works. Anyone care to try and offer an explanation?