How does Lee use Scout's innocence as a vehicle to explain the prevailing racial tensions in post civil war America in the south?

Atticus has a simple and beautiful way of explaining racism. Because Scout is a child, Atticus is free to use his simple but effective logic. It is a sort of logic that a child can understand but with a sense of depth and compassion that any adult can relate to. It is Scout's innocence and empathy that enables scout to illustrate the utter nonsense that racism is based on.