Try rephrasing the question or clarifying that you understand it before answering. That will give you enough time to start thinking through your answer on the sly. It could also be that the question wasn't clear,. i.e. you know the answer, but you just didn't quite understand the drift of the question itself. If not, it should go without saying that you shouldn't lie. Fess up. But don't say "I don't know" until you absolutely have to.