Remember you are are selling your self.  When the interview asks a question, think about the important information you need to get across.  It is an oppurtunity to expand on an area of knowledge in your background.  Saying you don't know something is always an option, never hurts to add how you might get the information too.  Just don't spend time dancing around a topic, avoid answering in general terms, saying "you know" is never good.  How is the interviewer suppose to know?  In most cases you just met, there is no shared history, no common referance, the whole point of the exercise is the interview!