I present to you a riddle:
What is the best and worst thing you can say in an interview? (Hint: they are the same thing).
If you just said "I don't know," then you are correct!
Simply saying "I don't know" in an interview is a conversation killer. This phrase is a dead end that not only amounts to admitting defeat, it shows no initiative. Remember that an interview is more than just answering questions, it's about telling the interviewer what you do know and what you can find out. "I don't know" leaves the interviewer no choice but to move on with the next question to find out what else you don't know.
If you truly don't know an answer, the best thing you can say in an interview is the truth plus further explanation: "I don't know, but I DO know…" or "I don't know, but I would find out by…" Following up with additional information shows that you can be honest about where your skills are currently but you also have the tenacity to learn new ones. It's even possible that the query was designed to see how you would respond to a question you couldn't answer. Nobody knows everything, and you don't want to pretend that you do.
Job interviews aren't true/false tests. Employers are looking at the skills you already have, but they're also looking for creativity, attitude, and more. Don't worry if you don't know one answer, just do your best to steer the conversation to your strengths and show them your initiative.
Of course, if you find yourself saying "I don't know" to several questions in a row, perhaps you are interviewing for the wrong job.