Occassionaly a student asks a question during lecture which I'm not only not expecting but am not prepared to answer. I feel it is OK to admit it if you do not know the answer. I usually tell the student, that they have brought up an great question or interesting topic to research. I tell them I'll make a note ( I make sure to go to the podium and write it down right then. That way they know I took them seriously and will follow through.) and tell them I will research the information and see what I can find. I thank them for bringing up the question and occassionly mention their question/topic might be a good addition to the lecture.
I make sure to follow through and provide the class with an answer as soon as possible.
