“Teachers Pay Teachers contends that it hit a milestone last year, when its 80,000 contributors earned more than $100 million, and that at least a dozen have become millionaires since the site launched a decade ago. Other major sites including Teachwise and Teacher’s Notebook, and recently such corporate players as Houghton Mifflin Harcourt and Amazon, have launched sites of their own. But some educators worry the increasing monetizing of lessons will stifle the longstanding practice of teachers freely sharing their ideas. And legal experts question whether teachers actually have ownership of the lessons they are selling.”