Any magicians ,or regular people n_n, out there willing to explain?

Question

I know how to make this card trick happen, I just don't know WHY it happens. . . Could anyone explain it to me?

Count 21 cards out of a normal deck. Lay out three stacks of seven cards each, face up, asking someone to pick out a card in their mind. After all 21 cards are laid out, ask them which stack the card they chose is in. After they tell you, pick the stacks up one at a time, turning them face down as you do. The stack they chose must be placed in the middle of the other two stacks. Do it twice more, each time placing the stack which contains their card in the middle of the other two stacks. After the third time you have collected them, turn the cards face up one by one from the top of the stack. . . And their card is always the eleventh one you turn up.

Sorry if I didn't explain this very well =( My apologies again if the solution is painfully obvious [which it probably is.] Thanks for any help you can give! Have a good day.

-Cassandra

Answer 1

I learned that one when I was 14. Its a good one.

Answer 2

It's because of how you deal out the cards. Dealing out the cards one card at a time, row 1 row 2 row 3, you change the card's position so that it ends up being in the exact middle of the 21 card deck.