Robert,
The thought of not being able to answer a question is a scary one for some faculty ad staff members. It happens more often than you might think. Sometimes, you simply don't know the answer. In other cases, you may know, but your brain freezes.
Being prepared for not being able to respond can help alleviate some of the anxiety, and help you make the most out of a difficult situation.
Perhaps the most significant thing you could do if you have been totally stumped by a student's question is to research the answer and then include that answer… >>>