Who explained to scout what it means when atticus says it is a sin To Kill a Mockingbird?

When Atticus purchases Jem and Scout air rifles. He instructs them to use care and responsibility when using guns. He warns that they should never kill a mockingbird. His reasoning was ñMockingbirds don�t do one thing but make music for us to enjoy . . . but sing their hearts out for us. That�s why it�s a sin to kill a mockingbird.î