You're at the library, borrowing a book that you plan on highlighting and taking notes in. It's not uncommon for others to see your doodles and thoughts in the books you return. This time, you notice another person in the library, Liu, taking out the same book you just returned, and reading your notes and highlights. You think nothing of it at first, but as you continue to borrow and return books, you notice that he's been keeping track of your notes and learning more about you as a person.