A textbook’s blank margins offer more room for disagreement than its answer key.

The answer key ends the discussion and the margin begins one. Everything a textbook presents as settled was argued over, and the blank space beside it is where a reader can push back, note a doubt, or record that this does not quite follow. The book offers conclusions and the margin offers participation.

Write in your books where you own them, especially the disagreements. Going back to those notes later is more useful than rereading the text. And with students, asking what they doubt about a chapter is a better question than asking what it said.