In chapter 12 of To Kill a Mockingbird, Cal takes Scout and Jem with her to church. Atticus has to go out of town for business with the State Legislature, so he leaves Cal in charge of the children. The kids are excited to go to church with her, but when they get there, they see that Cal talks differently to her black friends than she does at their house. Jem and Scout are surprised to hear her talking like she is.
"That Calpurnia led a modest double life never dawned on me. The idea that she had a separate existence outside our household was a novel one, to say nothing of her having command of two languages."
Scout finds it very curious that Cal lived two separate lives. Scout thinks that all people are exactly what they show you they are, yet with Cal, she sees that she has to live two very different lives. Cal teaches the kids that, being a black woman, she has to act one way while at work at the Finch home, but she also has to act a different way around her black friends.
"It's right hard to say," she said. "Suppose you and Scout talked colored-folks' talk at home it'd be out of place, wouldn't it? Now what if I talked white-folks' talk at church, and with my neighbors? They'd think I was puttin' on airs to beat Moses... It's not necessary to tell all you know. It's not ladylike-in the second place, folks don't like to have somebody around knowin' more than they do. It aggravates 'em. You're not gonna change any of them by talkin' right, they've got to learn there's nothing you can do but keep your mouth shut or talk their language."
What Cal is trying to tell the children, is that you have to speak the language that people understand. You can't go around acting like you are better than anyone else. You have to reach out to people exactly where they are. This is a lesson we could all do well in learning.