This is the question that Sergey Brin, co-founder of Google, would routinely ask people he was interviewing in the early day's of Google's existence.
It is an excellent question–and one I have recommended before–but, as this article points out, there is an additional positive side-effect of asking this in an interview setting. Even if you don't end up hiring the person, your time will not have been wasted if you learned something that you didn't know–or, better yet, unlearned something you thought you knew.
Interested in some other questions that might help you unlearn? Try these on for size: