Dirty eye glasses and dirty windows are pretty obvious to us. Our first impulse as adults is to clean them, to make them clear so that people can see out of them. We are not surprised when a student’s glasses are not clear, even if they were clear that morning when the student left home. Why should we be surprised when their understanding of something we just taught is not clear? So much of what is taught is not clearly learned. And I think you know by now that one of the fixed ideas, one of the familiar refrains, of this book is that school is not really about teaching-it is about learning.