What should I do when I don't know the answer in a technical interview?

The move is: honest flag → reason from adjacent knowledge → out loud. Interviewers score the recovery, not the gap. Script: 'I haven't worked with X directly — but based on [related thing I do know], I'd expect it works roughly like… am I in the right area?' Why this wins: real engineering is mostly navigating things you don't fully know. Watching you reason from first principles under uncertainty is MORE signal than a memorized answer. Some interviewers deliberately push until you hit unknown territory — reaching your edge is the plan, not the failure. What actually fails candidates: bluffing (they always pull the thread and it always unravels), silent freezing (unusable signal — think out loud even if wrong), and spiraling ('sorry, I should know this' — once is human, repeatedly is a red flag about pressure). One unknown answered with structured thinking routinely beats five known ones delivered flat.