This was a fun problem. On the other hand, I felt cheated when Vidit Drolia of Yammer gave this problem and expected it to be common knowledge enough to be solved in 3 minutes. Don't think it's quite that much of a valid interview problem.
Puzzles during the interview -- show others how smart you are by giving clever puzzles. Watch them struggle, make them feel bad for not figuring it out under pressure. Also accept lots of people who can google puzzle answers, or just are really good at solving puzzles. Which, works great if they are running a plumbing company and have to figure out good shapes for your manhole covers, a prison, or are just evil and like to shrink people and throw them into blenders.
