Difference between an Optimist, a Pessimist, and a Realist

OK, try this one on for size:

In a room, on a bar, sits an ice cold cup of soda, half full.

An Optimist walks in, sees it, and thinks, "Aah. The glass is half-full. It has the potential to be quite refreshing."
A Pessimist walks in, sees it, and thinks, "Rats. The glass is half-empty. There's not enough in it to quench my thirst."
A REALIST walks in, takes one look at the glass, and screams out, "OK, who's the wise guy that drank half my pop?"