Jonathan, I think of back up plans as a tool to use to avoid wasting the students time. Current and culmulative experience in a given profession are always a valued asset, enabling Q&A to occur seamlessly without much distraction from the focus of the class. I always follow a students question with a statement like "great question" or "interesting question". Usually my response is one of three. I may be able to quickly answer and move on. Or, the answer may take more time, and I'll ask the student to stay after for a thorough response without holding up the whole class. Or, if I'm not sure I can answer, I'll so state, and make sure I follow up up after I determined the correct answer. Barry Westling