When does the penny drop for your reader?

What we can learn about narrative tension from C S Lewis in Narnia

Many years ago, a writer friend who had taught secondary English told me a story about his Year Eights (12yr olds). They were reading C S Lewis’s The Voyage of the Dawn Treader and had reached the scene in which Eustace finds a cave full of dragon’s treasure and tries some of it on but, being very tired, falls asleep (pp 43-45 in my copy).