What to do when you can't answer a question

I learned the solution when I was talking on Radio London at midnight about etiquette. I was asked a question about etiquette, when Americans eat forks in their right hand, and I could not answer. Later an American girl phoned in to say she did not have a question but she could answer the previous question. Then I got another question about American etiquette and I said I also wanted to know the answer and I asked if any American who was listening could give us the answer.

From that I learned that you as the expert don't want to leave the questioner with no answer if you can find an answer. If nobody in the audience can answer, then the fact that you can't either doesn't look odd. You can offer to research the subject and thank the questioner. 

If you get an answer from another person in the audience, that adds variety.  So long as you or the chairperson get the last word giving a happy finish.