When you have no clue about the question, how do you answer/act when you do not know the answer at all? Telling the truth is pretty obvious. But how could you try to transform this weakness into a strength?
if you don't know anything about it, the best thing is to be transparent and admit that. this way you don't waste anyone's time.
but, you better know a little something about everything: and introduction or what's it about or at least the domain to what that question/technology belongs to or what's the reasoning behind the existence of that technology.
show the interviewer you know the world around you.
on the other hand, if you do know a little bit, start from there.
most interviewers will help you, and even provide and explain the answer. this is where it's important to not just play dead. ask questions and try to understand what he/she's saying. make sure you leave that interview with something new.
some people do care about your ability to understand things and about your attitude towards new stuff
When I really don't know the answer: "Honestly I don't know. But I'd be very interested to hear the answer."
It's a great way to bound with the interviewer since he/she is now the one under the spot. It shows that you're honest about your skills and eager to learn. Plus if the interviewer has difficulty answering his/her own question in a concise manner it may demonstrate the question isn't trivial.
I feel a really important aspect to these questions is elaborating your thought process. I know that a lot of the interview questions I've had were purposefully designed so that it would be unlikely that I know the answer immediately.
I feel that it is very mature to admit that you do not know the answer, but you should follow through with a methodology for going about trying to solve the problem, just to show your analytical skills.