What to do when you don't know answer to question asked by interviewer?

Okay, now let's look at this situation closely. There are following possibilities -

Situation #1: You know something, but not very confident about the answer:

This is a typical situation. The interviewer asks you a question and you aren't very sure about the answer. But of course you know 'something' very relevant. How'd you handle such a situation?

Well, the answer is simple: First decide how much you know and whether your answer will add something valuable to the conversation between you and interviewer. If you are absolutely sure that 'that something' will add value then politely accept that you don't know the answer but know something that is very relevant. You can use following words -

"Sir/Madam, I'm not familiar with <> but I know that it involves <> "

OR

"Sir/Madam, I don't have knowledge about < ... > but what I know is < .... > "

Look, the point here is not to tell the interviewer how smart you are, but to inform him/her that you're aware of the topic and can 'add' something that drives the conversation forward.

Situation #2: You're absolutely blank. Don't have a clue about what the interviewer asked:

This situation is not abnormal. Many times, the interviewer may ask you to see how you handle difficult situation. When interviewer asks you something you've no clue about, don't be ashamed to say,

"Sir/Madam, I'm afraid, I do not know < .... > "
"Sir/Madam, I'm not familiar with < .... > "
"Sir/Madam, I'm not sure whether this is about <> "

You may ask question to the interviewer just to be sure that you've understood the question correctly. Don't be a rat and just go on babbling about every damn thing you know in the world; just to appear 'smart' and 'employable'. You'll get rejected.

Situation #3: You know the answer, but don't recall it properly:

See, your promptness in answering does not make you look smart & intelligent in any interview. Many candidates think that if they start answering as soon as the interviewer has done asking his/her question, they'll look smarter. Wrong!

It is ABSOLUTELY OKAY to take your time before you answer. But not too much time. 10 seconds is enough time to gather things in your mind before you speak out. If you need more time to collect the things, request the interviewer to grant you more time; they'll be happy to grant you time than listening to incorrect answer or your mumble.

Remember, use your mind when you handle such situation. For example, if you're a fresh graduate appearing for interview and the interviewer asks you a definition or question based on your engineering discipline; you can't say "I don't know", because you can't take chances with that.