It's rude to ignore the rules in the library.
Charles is in the library with a friend, working on an assignment together, when Charles sees a mistake in his friend's paper.
Charles wants to notify his friend about his mistake.
Charles points at his friend's mistake, then shows him his paper with the correct work.
Charles tells his friend that he messed up and describes the problem to him.
Charles' friend corrects his work and no one is disturbed.
The librarian gets annoyed and tells Charles to be quiet.