I would often ask them to describe operations that they take for granted. Ask a girl to write a paragraph on how she braids her sister’s hair. Ask a boy to explain a sports rule. These are just basic starting points, where students learn to write on paper what they might tell a friend. It keeps it concrete. If you ask a kid to write on “My Favorite Movie,” you’re opening the door to subjectivity, and hence to a flood of clichés