I have heard it said that you learn more by teaching something than just by learning or doing it. As I was preparing today for a lesson I caught some new insights into why that is. For one thing, the time taken to prepare what you are going to teach is much longer than the time that will be taken by a student in the lesson. Another factor that makes this true is that what you end up teaching in the actual instructional setting is not precisely the same as what you had prepared in advance. The teacher benefits from the preparation as well as anything new that comes in the lesson delivery. The students only get whatever came during the delivery time.