A spectator freely chooses a card from a shuffled deck. The magician folds the card in half, then in half again, and in half again, until it vanishes. The magician then produces a lemon and a knife. The lemon is cut in half and, lo and behold, the selected, folded card is inside!
Although the deck really is shuffled, the card choice is not as free as it appears to be. While fanning the cards face up to show that they are all different, the magician looks for the card that is to be forced. When the card appears, the magician asks someone in the audience who is standing five or six feet away to point at a card. "Ah, this card," the magician says, taking the forced card from the deck. Because the spectator is so far away, they can't really see the cards all that well and just have to take the magician's word that this is the card they were pointing to.
The balance of the deck is put away and the magician shows the chosen card to the audience. The card is then folded in half until it is so small that the audience can't easily see it (about fifteen folds) and casually dropped on the floor. (Note that Mallusionist.com does not endorse littering. Magicians must clean up after their act, whether they've dropped cards, sprayed confetti, or botched an assistant bisection.)
With the card gone, the magician takes a lemon and knife from some secret location. The lemon comes from the magician's own lemon tree and has been specially prepared. The magician must keep close watch on the lemon tree and, when it begins to flower, staple folded playing cards to the forming buds. As the bud grows into a lemon, it will encompass the card, becoming a lemon with a card in it. (Tip: Use dissolving staples so that anyone attempting to eat one of the special lemons won't get a mouth full of sharp metal.) (Another tip: Make sure you don't forget what card is in each lemon, either by using a Sharpie to write the identity of the card on the lemon's branch or buying a bunch of decks and using the same card in each lemon -- the leftover cards can be donated to a magician's charity.) The lemon should be picked the night before the performance, and it should be ripe but still firm.
The rest of the performance is just acting. The magician cuts open the lemon, produces the selected card, receives applause, and makes a few extra dollars selling "magical lemonade" after the show.
Note: The same technique can be used to perform Dollar Bill to Lemon (just write the bill's serial number on the branch or always use bills with the same number). It can also be used for Signed Card to lemon, but waiting for the lemon to grow around the card may cause an unacceptable delay in the performance.