If I were interviewing a candidate and had decided to ask this question, my response following their reaction would be "Just kidding. We don't do that here" followed by taking note of how concerned they initially appeared at the question.
The "just kidding" would probably need to be before I answered. If I start to get up and walk out and then they say "no, I was just yankin' ya!" I'm going to be really suspicious.
What exactly do you think their level of visible concern would tell you about them? An employer conducting jokey personality tests would be a huge red flag for me.
