It is disrespectful to vandalize library books.
Ace and his friend are at a library and are bored of reading books, but won't be picked up for another hour.
Ace wants to do something fun and different.
Ace asks for a piece of paper from the front, then plays pen-and-paper games with his friend.
Ace starts drawing in the margins of the library book to make a stop-animation drawing, then shows his friend.
No one else in the library is disturbed and all that is marked is the piece of paper.
The next person to check out the book reports it to the library, who then has to fix the book.