One of the most powerful phrases a teacher can use is, "I don't know." When a student asks a question, if you are not sure of the answer, or even just simply do not know, the best response is always an honest one.
Often, teachers feel the need to be the 'know-it-all' in the equation, and ad lib answers. When those answers are inaccurate, you've done your students a major disservice. Let go of the feeling you must know every answer, and gain respect from your students. Find out together. Challenge them to find the answer on their own.
Regardless, do not ad lib, chancing you will give wrong information. This misinformation will be the one tidbit of knowledge they remember from the day.