Super Lemon by Alex Ng

You first casually ask your spectator to name a random fruit. Regardless of what they say, you proceed to take out a lemon and perform a lemon trick. With just a lemon and a knife, you invite everyone in the room to participate in a game of imagination. 

Within the entire process, everyone is convinced and swears you are holding a lemon and a knife. But as the routine continues, it is revealed that the knife and the fruit have magically morphed into a fork and a yellow paper ball, causing the spectators to question what they just witnessed. 

As the kicker-ending, you open the paper ball, and what's written inside, is the SPECTATOR’S RANDOMLY-NAMED FRUIT. A shocking ending for a visual mentalism opener!