Half full?

My dad once had a job interview where the interviewer asked him if a given glass was half full or half empty. He said that he didn't know.

"What do you mean, you don't know?"
"Are you currently in the process of filling it up, or emptying it?"

I'm going to go out on a limb and say that that is the best possible answer to that question, despite it meaning that my dad didn't get that job.