I don’t know, are the three words that I would like to hear from interviewees, when then don’t have an answer to an interview question. It's much better than rambling on with answers that absolutely make no sense. Really.
Beware! Show some discretion (Read: common sense) in using them for questions for which you really don’t have an answer. Those questions don’t include, “Why are you looking for a change?”, “Why do you want to work here?” or something along the same lines.
Fear of getting rejected at interviews or even getting judged, puts undue pressure on some candidates to come up with answers that absolutely make no sense. While on the contrary, the very reason to reject that person would have been the answers. A good interviewer isn’t expecting you to know answers to every question. They only try and check your approach towards a solution for a certain problem (or question). The way you approach a problem will tell a lot more than the actual solution itself.
Now that you have told, “I don’t know”, what’s next? There is a couple of ways that might help you gain the interviewer’s confidence.
First, ask for the answer. In my experience, I’ve noticed that candidates tend to go on the back-foot or get defensive or go into a shell, when they don’t know an answer. It’s that awkward minute between an unanswered question to the next question that you should grab with both hands. Ask for the answer. Yes, it’s that simple. You could say, “I don’t know, but I’d like to know the answer to the question. It will help me understand the topic.” You’ll be surprised that a trained interviewer actually looks for such traits in candidates.
Second, don’t use the clichéd, “I don’t know the answer, but I’m a quick learner.” It means zilch to a person trying to determine your approach. In case you are uncomfortable asking for the answer during the interview, it’s ok. Make a note of that question. Get back home. Dig for answers, either through books, blogs, online, whatever it takes to get the answer. Mail it to the interviewer with mention of the sources you used to get to the answer. This is an approach that very few candidates take and it could help you stand out in a crowd. Your efforts will definitely be recognized by good interviewers.
Saying “I don’t know” is one of the easiest answers for a question that one doesn’t know the answer to. It’s what you do about the don’t-know-part that counts.