Presenter notes: Open by admitting the difficulty from the last lesson rather than pretending it did not exist. Survey questions had four groups to keep track of and most of the class lost one somewhere. Say that today nothing new is being learned; the same four groups are simply going to be drawn as four patches of paper, and a patch cannot be forgotten because you can see it sitting there empty. Framing it as a fix for a real problem gets far better attention than presenting it as a new topic.