"I don't know but I have an idea how to find out", just be honest and enthusiastic about learning

Admit that you don't know the answer they are looking for then do one or both of these things:

1. Ask clarifying questions to get to the root of the problem, not the solution they're asking for. Then provide them with a different solution you know enough to create which solves the same problem.
2. Talk about a similar scenario you came across in the workplace where you did not know something you needed and what you did to eventually solve the problem.

Do you know technology X

Always be honest. If you have at least heard about technology X, you could say: "I haven't used technology X myself. From what I've read about it, it's very similar to technology Y, which I do know."

If you've dabbled in technology X, you could say: "I have experimented a little with technology X. I feel I could come up to speed with it rather quickly."

The purpose of an interview is to see if you fit with their team and if that team will fit with you. You might be given questions for which there is no real answer. They want to see how you will behave under pressure, so they may artificially induce that via bizarre questions. In addition they want to know if you are teachable, and if you are smart and think on the fly. Be super honest because claiming you know something you don't, then falling on your face about it is much worse than being honest. Knowing your limitations and being enthusiastic about learning new things is much better than being fake about knowing some technology.

That being said - why are you interviewing at some place if you don't know the technology they work with? Do some research ahead of time and at least get some kind of working knowledge. Show that you are willing to go the extra mile to get this position.

And, as I said, it's about whether they fit with you. If you end the interview uncomfortable and out of sorts, then that may be a sign you do not want to work with these people on a daily basis.

The simple answer if you don't want to say "I don't know" you can say "That is a good question, I don't at this time, but I am happy to learn about it and take a look." Simple ways of saying, "sounds interesting" and affirming that you are interested in learning about it.

I don't know what a developer should do, but here's my process:

How do you implement X in lang Y? - Work with the interviewer as if you're pair programming. Even if I totally have no idea about something, I usually say I "don't remember" things about it and come up with some educated guesses about it based on the context they're giving me. Then I start from those assumptions in order to define "X" in great detail, confirming each piece with the interviewer as I go.

If I do it right, the process of discovery/learning from my interviewer looks identical to when I already know the thing, because either way, I am taking care to confirm all my assumptions.

I also always make sure they provide at least a sample input/output so that I have an accurate representation of THEIR assumptions about its behavior. This is a place where newbies will often out-perform mid-level developers, because they're less inclined to make assumptions in an attempt to show off technical skills.

Do you know technology X? - This is an opportunity to have a conversation with the interviewer about what X is and what transferable skills you have from working with other, similar things. Talk about the features of X and how they compare to other offerings out there. Highlight anything that's similar, especially if you can show a deep understanding of the similarities between different technologies. For example, I've been known to talk about the transferability of React/RoR skills in someone with strong CS/OOP basics.

This is a great opportunity to learn more about technology X in general, too! By talking to your interviewer, you'll get a better picture of why they're an enthusiast about the technologies their company is using (or why they're not but it's still "good enough," which is SUPER useful information—nuanced opinions are usually the most informative).

TBQH, by FAR the most successful approach I've taken to interviews is to treat them as an opportunity to learn, and fully lean into the joy of learning.

Simply say the truth! "I'm not sure how to do that." If I were in this situation I would also add that I would do so and so to find the solution. This shows your problem solving ability.

You should be honest! But don't just say "I don't know the answer."

Rather take your time, analyze different approaches to answer. If still nothing! then it's time to come clean and say "It's a good question, but I'm sorry, I don't have the answer off the top of my head. I will be sure to follow up with the answer after the interview."

Be honest and motivated. Show that you are eager to learn the ropes and whatever else you need for the job. If you cannot answer a question right now, just tell the interviewers, but hint at how you will overcome the problem. No one can know everything, but it is very important to be able to solve the problem given nevertheless!

• "I do not know tech X / lang Y, but I am motivated to further my horizons and confident in my ability to learn fast." Possibly add "I already know a few similar technologies, which will help me get productive in a short amount of time."
• "In order to implement Z, I would require more time to think about an optimal solution and investigate possible algorithms and side-effects. In my opinion, a quick implementation results in suboptimal code, which has to be refactored later on or might lead to problems and bugs. Hence I do not want to give you a quick answer as it would not be representative of my full capabilities."