A colleague who used to be a maths teacher pressed a book upon me and I am really enjoying it. The Princess and I were out for a cup of tea and each of us was reading her book. I looked up and said to her, “I’m really enjoying this, the author says that finding something in maths is like discovery not invention. Isn’t that clever? It’s out there waiting to be found. And you can see that’s true. I mean look at pi. It’s a constant for any circle and it’s true for every circle even if we didn’t know what it was. Isn’t it fascinating?” Pause. “Do you know what pi is?” Herself, coldly, “No, I am only 9, you know.” I returned to my book, suitably chastened.