In To Kill a Mockingbird, according to Atticus, it's a sin to kill a mockingbird. Scout asks Miss Maudie why. What does Miss Maudie tell Scout?
The answer to this question can be found in Chapter 10, when Scout becomes ever-aware of her father's increased age and asks Miss Maudie about the one thing that she has been told by Atticus it is a sin to do - to kill a mockingbird. Miss Maudie responds as follows:
"Your father's right," she said. "Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't next in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."
They then go on to discuss the neighbourhood and the age of its inhabitants, and this allows Scout to talk to Miss Maudie about her concerns about her father's "old age" from her perspective. Of course, when thinking about the mocking bird in this excellent story, it is important that you think about how it operates symbolically - that is, how it stands for something beyond itself rather than just a literal mockingbird. Therein lies the genius of the title.