I've asked interviewees very difficult questions for this exact reason. I want them to say "I don't understand" or "I don't know" rather than bullshit me an ans

I've asked interviewees very difficult questions for this exact reason. I want them to say "I don't understand" or "I don't know" rather than bullshit me an answer. Of course, after saying "I don't know," I instruct them to elaborate on how they would work out the solution -- co-workers? Google? Experiment?