[fredophile]

A good way to think about how to answer is to look at it from the perspective of the interviewer. What information do they want to get from you by asking this question? I'd say this question is aimed at finding out how you react when you're challenged. Can you describe what made the problem hard? Maybe it had conflicting constraints or goals. Maybe you were debugging some particularly tricky problem. What did you do when you ran into difficulty? Did you throw your hands up and give up? Did you talk with teammates about potential solutions? Did you have a systematic approach or were you just trying random ideas to see if one worked?