A child can finish a book and begin an argument that lasts decades.

A book read at ten can start something that runs for decades. The child does not finish it and put it down so much as begin a long disagreement or a long attachment, returning to it at twenty and forty with different objections. What looks like a completed activity is the opening of a conversation.

Give children books slightly beyond them and let them argue. Notice which books you are still arguing with, and consider rereading one. The ideas that stay with you are usually the ones you never quite settled.