Thanks for your comments! I think it's important to recalibrate your expectations as an interviewer. Interviews are stressing and people are nervous and never themselves. If I was just chatting with a person informally over coffee or beer, we had unlimited time, and it wasn't going to possibly decide their entire career, I think most people would ace this question easily. But in an interview setting, those same individuals will struggle. It's up to me the interviewer to appropriately set my expectations, and try to make the best out of a kinda-crappy and stressing situation! :)