Picture two books sitting on a shelf. Picture the bookworm starting on page 1 of the book to the left (page one is the page all the way to the right!) and eating his way through to page 400 (page 400 is all the way to the left of the book to the right) When he's done, he's eaten through exactly two covers, no pages at all. Set up two books. Try it out.
"I knew that."