"That's a very (interesting) (intelligent) question! I have myself thought about it, but I don't think anybody has the full answer. I am working on it and I'll get back to you as soon as I have one."
"The answer to your question is very well known, but it will take quite a bit of time to repeat it here. If you are keenly interested, you can meet me after this talk, and I'll give you a URL where you will find it. Or you can Google for it."
"I have already dealt with in the first few minutes of my talk. You were probably not very attentive. I don't want to waste everyone's time repeating it now."
"Sorry, I can't answer that question. If I do, I will be revealing (a lot of classified information that impacts national security) or (some trade secrets that will have adverse effects on my company).