palm reading
Offer to read someone's future from their palm. It all starts seductively promising, with things on the palm resembling aspirational object that the person will one day own.

"You're going to own a mansion shaped like a finger."

After two or three predictions, hawk up a huge greenie into their palm and inform that that it is their swimming pool. Luxurious.

Actually, this trick was seen on The Simpson, episode 9F06, New Kid on the Block, along with Wet Willies. Is it funny if it's been on The Simpsons? Probably not.
written by excluded pupil, approved by Log

Also available as the punchline to the question "do you want your palm read?" is the 'classic' daubing of the outstreched hand with a red felt marker.
written by excluded pupil, approved by Phil

A good comeback to someone playing this trick is to approach them and ask 'can you read palms?'. Thinking that you have walked right into their hands, they will reply 'yes'. You then reply 'read this then', at which point you extend your palm to reveal the words 'YOU ARE A COCK'.
written by an*nymo*s *se*, approved by Phil