If you consider yourself an expert & someone asks you a question you don't know the answer to, you've got three options as I see it.
You can start by overwhelming them. To do this you usually need to know some technical words that are generally associated with the topic they asked you about. If they're sincere & you see confusion on their face as they try to follow along with your answer, the trick is working. You then either have to make a quick exit immediately following your answer or change the topic of conversation away from their question by asking them a question that's related to one of the big, technical words you used: "How about you? Have you ever pondered the salmonidical mindset as it rises for a satiating meal of ephemeroptera?" This should avoid you having to state that there is something that you, in fact, don't know.
The second option open to you when asked a question you can't really answer is to answer, but not the question that was asked. You answer a question that you do know the answer to, making sure that it's somewhat close to the subject of the original question. Politicians do this all the time which leads me to believe that it must be an easy technique to master. For example:
(upon approaching the stream with your fishing partner)
You: "This looks like good dry fly water."
Partner: "Which dry fly would you use here today?"
You: "I've noticed that after the first full moon of the month trout feeding activity is less on this stream."
The third option you could take when asked a question you don't know the answer to is to simply say "I don't know." Of course you would then begin to discuss with the questioner where or how the answer might be found. Perhaps you might even suggest that the two of you investigate it together.
Of course the option you choose depends upon your personality, ego, & a bunch of other things that make psychologists money. Just be careful. If you choose one of the three options too often, folks might stop asking you questions. Then what kind of expert would you be?