The best way for the student to retain the content he has just learned is to explain it, however partially or imperfectly. Teachers and students know this, which is why it is common practice. However, it is also clear that in doing so, if one doesn’t see it as a mere exercise, one risks arbitrating on knowledge that has not yet been consolidated, and naturally one makes mistakes, and a lot of them. The curious thing is that this seems to be a necessary process, and we often come across the connoisseur who, years before, had wandered far from where knowledge had taken him. Mastering a subject is perhaps no more than intimately gathering the arsenal of errors that surround it.