What a great point! 15-18 minutes is not very long. My lectures tend to be very long and intense. Once I see my students fading, I like to stop and walk around to help them one on one before going on to the next topic. Sometimes it is difficult to cut a topic/lesson short, especially when it is a intense software program. I have learned that sometimes it is better to finish, even if it is long and cover it again for those who got lost. This reinforces the advanced students learning while catching the rest of the students up.
