Saw this in an article about Google from last year: http://online.wsj.com/article/SB1000142405270230491110457644...
About Sergey Brin: "I'm going to give you five minutes," he told me. "When I come back, I want you to explain to me something complicated that I don't already know." He then rolled out of the room toward the snack area. I looked at Cindy. "He's very curious about everything," she told me. "You can talk about a hobby, something technical, whatever you want. Just make sure it's something you really understand well."
Really like this. Covers a lot of bases in a single question, such as curiosity, depth, interpersonal skills, etc.