Set the trick up. Carefully cut off the green part of the lemon. Inside, you will find a small enough space for an extremely rolled up card to fit inside. Pick a card, we will say the Ten of Hearts, and roll it up, placing it inside. Using clear glue, glue the green part back on.
Remove the Ten of Hearts from the other deck. Place both decks into your pocket, making sure to differentiate between the two. You must know which has the Ten of Hearts and which does not.
For this to work, the Ten of Hearts will need to be "forced" onto your spectator. I wrote a tutorial on an easy way to "force" a card; so read that. Essentially, you need to make the spectator think the Ten of Hearts was randomly chosen, when in fact it was chosen because you made her choose it.
Ask her to memorize the card. Have her place it back into the deck.
Take the deck and place it into your pocket. Casually approach the area with the lemon.
Ask her to name the chosen card. Upon hearing the name, peel the lemon open and remove the rolled up card. Unroll it to reveal the Ten of Hearts!
Remove the second deck from your pocket and show that the Ten of Hearts is now missing. Since you placed the first deck into your pocket earlier, she will assume it is the same deck, when it is a different one.