Getting to an absolute "I don't know" does indeed take too long, but doesn't really do much for me as an interviewer anyway. My goal in these questions is to ge

Getting to an absolute "I don't know" does indeed take too long, but doesn't really do much for me as an interviewer anyway. My goal in these questions is to get to "I don't know, but I think... and I would verify by...", "I don't know, but one could try..." or "I don't know but I would Google for..." are all excellent places to end up. You're not BS'ing me, but you're not stumped either. You know sensible next steps. We all have to look stuff up regularly or test the unknown. Can be a problem trying to out-generalist a generalist, but if you pick something in your niche (I ask them lots of other questions relating to their background, so this question doesn't have to cover the exact job requirements) and practice the problem with dozens of interviewers, it'll be pretty hard to have someone who can genuinely out-think you in the depth of that problem. And missing out on the "I don't know" is a small price to pay to find a candidate that does that well on the problem!