In chapter 17 of To Kill a Mockingbird, Atticus is questioning Bob Ewell. Atticus asks, "Mr. Ewell, can you read and write?" The question makes Scout nervous. As the child of an attorney, Scout is privy to certain aspects of the profession that others may not notice. While she is curious to know the purpose behind the question, she is concerned because Atticus is asking a question that he does not know the answer to. According to Scout, one should "never, never, never, on cross-examination ask a witness a question you don't already know the answer to." The result can be unpredictable. In this case, however, it proves to be in Atticus's favor.
Bob Ewell writes his name with his left hand. Additionally, Sheriff Tate says that Mayella's right eye is blackened. Scout considers this information and concludes that the person who hit Mayella must be left-handed. Later, when Tom is asked to stand, Scout notices that Tom's left hand "hung dead at his side." His left arm was damaged in a cotton gin when he was a boy, meaning it would be extremely difficult for Tom to blacken Mayella's right eye.
Scout thinks that Atticus is wildly asking a question without thinking about the answer Ewell could give. She has learned from a young age that when you cross-examine some one, the lawyer must not "ask a question that you already know the answer to."