As my sister reads books, she regularly highlights sentences and jots down notes in the margins. One day, while riding a train, she was underlining a passage when she overheard a mother reprimanding her child for doodling in a book. My sister quickly put her pen away, not wanting the toddler to ignore her mother’s words by following her example. The child simply wasn’t capable of understanding the difference between a book loaned from the library and a purchased copy—the difference between vandalism and making personal notes in the margin.