If you really want to screw with someone...(along the lines of what OPD mentioned)...
Tell them that as part of your psychiatric training you can read minds.
http://www.indianchild.com/strange.htm
Do the trick in the above link. I've found from personal experience that an overwhelming majority answer carrot. I don't know if its 98%, but it's a majority, enough to make this trick worth it.
Then, after you tell them you know they thought of a carrot, start going a bit deeper.
E.g., start telling a guy, who is not homosexual, that you are noticing some homosexual thoughts. Mention a famous handsome celebirty that you know the guy's mentioned at least once.
e.g. If the guy's a football fan, mentioned the quarterback of his favorite team.
"I know you've been thinking about this guy. I'm detecting there's something rather deep about this."
Be careful with this type of joke. It is pure evil in the wrong hands. Do it only to someone you know can take a joke.
When I was a psychology major in college, I did the old "I'm a psychologist, I can read your mind, to a buddy of mine who was homophobic. To make a very long story short, he actually started to believe he was homosexual, based on some very innocuous and irrelevant data (ever notice you're the only guy on the floor with red shorts? Think about that. Red is the color of Mars, the ultimate sign of masculinity, and you choose to put red around your genitalia and buttocks. Think about it. No other guy on this floor had red shorts except for you, but plenty of girls in the dorm have red shorts. Think about it.)
Later that night he was about to call his parents to out himself, and he was questioning his sexuality. I'm not joking. A very evil joke, though at the time, me being 18, I rationalized it was alright because he was trying to get the floor to bully someone he thought was homosexual.