"I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do it — seems that only...

This quote is on page 285, near the beginning of Chapter 22 of To Kill a Mockingbird. Atticus has just returned home after the devastating verdict in Tom's trial — Tom was found guilty despite Atticus's clear demonstration that Bob and Mayella were lying. Jem was particularly optimistic about the possibility that Tom would be acquitted, and he asks Atticus how the jury could have ruled as they did, knowing that Tom was likely not guilty of the crime. This quote is Atticus's response, and it demonstrates that he is as disappointed by the verdict as Jem, and he does not know how to explain the racism that informed the decision to his children. The final line of this quote is especially powerful; it reflects a persistent theme of the book — the idea that children haven't "learned" to be bigoted. This suggests that racism is not a basic part of the human condition, but rather something we learn from our surroundings.