What page am I on?

Interesting discussion I had with some kids today: I was looking at a book that one of them had on her desk with a small corner ripped from a notebook page used as a bookmark. I was commenting on that when the girl sitting next to her (who I know reads a lot and is a smart kid) said she never uses a bookmark. "How do you know what page you're on?" I asked her. She gave me a kind of quizzical look, shrugged, and said, "I don't know" as if she'd never considered the idea before.