Scout teaches Uncle Jack to listen to both sides before punishing a child, and Atticus adds that you should tell a child the truth when he or she asks a question.
When Scout gets into a fight with her cousin Francis, Uncle Jack spanks her. When she is upset, he is surprised. In his mind, he warned her and therefore she should expect the spanking.
Scout is not upset that she was punished; she is upset that Jack did not talk to her first. He told her not to use foul language except under “extreme provocation” and in Scout’s mind Francis provoked her by insulting her father.
[You] never stopped to gimme a chance to tell you my side of it- you just lit right into me. … you told me never to use words like that except in ex-extreme provocation, and Francis provocated me enough to knock his block off-" (ch 9)
When Jack tells Atticus what happened, he agrees. He said Scout should have been punished because she used her fists and not her words, not because she used foul language. He also explains that Uncle Jack should tell a child the truth when she asks a question, because making a production of it just muddles them and children can see an evasion quicker than adults.