I'm an electrical engineer, and I have a similar question that I ask my entry level candidates - "How would you measure the voltage coming out of a wall socket?"

It's familiar enough to be accessible, but technically just far enough out of their reach that they don't know exactly how to do it. We talk about approaches, tools they could use, some potential problems or issues they'd need to overcome.

If they get flustered about where to start, I laugh, and say "Don't worry - I don't know the right answer. I'm actually afraid of AC mains electricity." Never fails to calm them down, because, well, it's kind of funny to hear an electrical engineer admit that they're afraid of electricity!

It's a question that I've gotten a lot of mileage on.