You have five cards; they have five cards. A mentally selected card from your pile magically crosses over to their pile! You now have four cards; they have six cards!
A deck of cards is shuffled, cut and mixed. You explain to your audience the reasons why they should never play cards with a magician. Offer to show them an example. Openly deal a face-up poker hand to one spectator and yourself. As they look over all of the cards, they see each card is obviously different. Collect the piles and give your spectator their pile to keep in a safe place. As you spread your pile of five cards, ask them to mentally select any one card and to burn that card into their memory. Once done, close up the spread. Have them reveal their thought-of-card for the first time. Slowly and deliberately you deal down your pile of cards one at a time. There are only four cards! Their selected card has vanished from your pile! Now ask them to retrieve their pile of cards. You'll notice how the expression on their face turns to one of disbelief as they begin to realize what might happen next! You deal their pile one at a time onto the table. Their pile of cards now has six cards. Yes, the selected card now resides in their pile!