The Joker: There are three kinds of people in this world. The optimistic that find the glass half full and the pessimistic that see it as half empty. Then there's the paranoid, and they just think that someone's drinking out of their glass.
Cop: Which one are you?
The Joker: I'm the one that knocks the glass over.
[Joker knocks over glass of water on the desk and giggles as the cops jump back with a shout]