What should I do if I can't answer an interview question?

If you have some guesses or related knowledge that you think may be useful, you can mention them, but make it clear that you aren’t sure. There is only one thing worse than being unsure - being sure and wrong.

First, don’t bullshit. I’ve only recently started interviewing people, and from the interviewer’s side, bullshitting really doesn’t look good for you, and they will always know. Remember, they are asking questions that they had already researched and know answers to. It’s not like they don’t know the answer either, and may end up believing you.

If you really have no idea, just say “sorry I have no idea”, and let them move on.

If you have some guesses or related knowledge that you think may be useful, you can mention them, but make it clear that you aren’t sure. There is only one thing worse than being unsure - being sure and wrong. This may prompt them into giving you hints and helping you along. It would also show them that you are humble and open to being corrected and helped. That’s a very desirable trait in candidates.