The problem is, that's not a particularly great learning environment for your student or yourself. I'm not talking about someone whose ego gets in the way of their listening and learning. I'm talking about a student that asks difficult questions. Someone that is clearly very mentally engaged in their training and likes to get into the nitty gritty of all things kung fu.
These relationships are integral for my students, and encourages me to learn and grow as an instructor. Therefore, I become a better teacher and listener. They also serve up some humble pie once in a while, when a question is asked that I realize, I don't know the answer to or have never really thought about. There are many questions I can answer with some listening, watching and attention to detail. But there are those that make you question your knowledge and understanding. And although most of the time I discover that I DO know the answers, it's again, refreshing to have to regroup and re-evaluate.
I think it's also important to sometimes say things like, "I don't know" and "let me think about that". It can be difficult to maintain confidence while telling your student you don't know something. But, this makes it clear that I'm not just carelessly answering questions to feel important. I am deeply involved in their learning and it's important to me.