Take a moment to think about the question. You can even say “Hmmm… that’s a good question. Let me think about that for a second.” Folks appreciate a little consideration when someone takes a question.
If you don’t have an answer be honest and say you don’t. The worst thing to do when you don’t have an answer is to b.s. the person. No one has an answer for everything, except a b.s. artist. So, feel free to say you don’t know –- folks find it refreshingly humble and honest. There are many ways to say this including: “I’m not really sure, I’m going to have to think about that for a bit and get back to you,” or “I’m not sure to be honest. What do you think?”
Feel free to think out loud and brainstorm with the person. You can do this by saying “I’ve never really considered that. Perhaps you can expand the question a little and we can explore it right now.”