Adult learners have an attention span, like everyone else and needs variation to stay engaged in the learning environment. To that end, I recommend The Rule of Twenty: "Ensure there is a change in classroom activity every twenty minutes." Whether it be lecture, lab, a video, whatever, the instructor should constantly plan to do something different every twenty minutes. To me, this reduces monotony and keeps things fresh and interesting. This also ensures an adequate amount of preparedness for the class because the instructor is only managing 20-minute blocks, which are a lot easier to predict than a 2-3 hour class session. -R