If I don’t know the answer?

We don’t know about everything that exists in world. There are penalty of questions, we don’t know their answer or we are unable to express in words. Normally in interviews there is a question where we stuck and unable to speak up. So what we can do to overcome in that situation as we mess up whole interview by unanswered only one or two questions. Here are some tips:

1. Be honest, if you don’t know about the answer then simply/politely tell interviewer that “I don’t know the answer” of this question.
2. It is perfectly acceptable to say "I don't know the answer to that, and I'm not familiar enough to hazard a guess."
3. It is also perfectly acceptable to say "I'm not entirely sure of an answer, but given my limited understanding of the topic, here are a few thoughts."
4. You’re better off admitting that than trying to bluff it and then getting caught. Of course, don't just leave your answer as "I don’t know", tell them how you'd find out the answer, such as "I'd search Google to learn more about this question and also consult a coworker who is more knowledgeable about this topic.
5. The worst case is being so thrown by the lack of answer that you stumble through the rest of the interview. Take a breath, put it behind you. It happens to everyone.