What's the first vegetable you think of?
Another peculiar trick I picked up from maths class...
Write down the word "CARROT" on a piece of paper and put it in your pocket. Then get someone to do some simple mathematics.
"Say a number between 1 and 10" is a good starter. Then get them to add, subtract, multiply and divide things to it. Get their brain working. Do a dozen or so calculations then suddenly burst in with "What's the first vegetable you think of?"
People will just about ALWAYS say CARROT!
Tip: Don't give the victim a chance to think, press them quickly and make them say the first thing that comes into their head!
Of course, the big finale is pulling the piece of paper out and magically showing you KNEW they were going to say carrot...