How does Lee use humor when Atticus talks to Scout about saying "nigger"?

I think the humour comes from the particular exchange between Scout and Atticus. Scout uses her intelligence and innocence to counter Atticus equally intelligent arguments that are seen many times in their loving respectful relationship,

"Do you defend niggers, Atticus?" I asked him that evening.

"Of course I do. Don't say nigger, Scout. That's common."

"'s what everybody at school says."

"From now on it'll be everybody less one-"

"Well if you don't want me to grow up talkin' that way, why do you send me to school?"