> I still don't understand how the effect was achieved. How can giving you
> cards make sure that the one you picked isn't there. My dad tried to
> to me but I still don't get it.
None of the original cards (including the card you chose) are in the new set of cards. Try doing it again, but before you choose a card, write down all the cards that you can choose from. Then, choose a card and look at the new set of cards presented (without your card in it). If you check carefully, you should find that none of those cards is the same as any of the cards you wrote down.
For example, they might present you with:
King of Hearts, Queen of Spades, Jack of Clubs, King of Spades
You choose say King of Hearts.
The next set of cards is
Queen of Clubs, King of Diamonds, Jack of Spades,
Your card is not there. Neither is any of the original cards from which you could choose. It's a good trick, and after you work it out you'll wonder how it ever fooled you. But it is really deceptive.