It’s great because there’s no “right” answer, but the process of getting to your answer is fun. Most interviewers just want to see that you aren’t thrown off by a difficult problem, and that you can begin to work your way through it, especially if you can raise possible concerns about what necessary criteria must be included in your solution.
My favorite answer comes from the movie Abandon, which is a horrible movie starring Katie Holmes that you should never watch. BUT her answer is cool. She’s sitting in a room with three people interviewing her, so her response is: “With or without us in it?” Delivered in a perfect deadpan: