In chapter thirty-one of To Kill a Mockingbird by Harper Lee, Jem is in bed recovering from the attack by Bob Ewell, and Scout, Atticus, Boo Radley, and Sheriff Heck Tate are on the Finches' front porch. Atticus is convinced that Jem killed Bob Ewell and begins to prepare his son's defense in his mind. Heck Tate, however, knows exactly  what happened and tells a different story in an attempt to spare Boo Radley from unnecessary attention.

In the sheriff's version of events, no one killed Bob Ewell.

Mr. Tate flicked open the knife. “It was like this,” he said. He held the knife and pretended to stumble; as he leaned forward his left arm went down in front of him. “See there? Stabbed himself through that soft stuff between his ribs. His whole weight drove it in.”
Mr. Tate closed the knife and jammed it back in his pocket. “Scout is eight years old,” he said. “She was too scared to know exactly what went on.”  

Of course, Tate knows exactly what happened. He understood that the reclusive Boo Radley killed Bob Ewell in order to save Jem's life; Atticus takes a little longer to realize this truth. Even Scout seems to understand when Tate makes it clear why he would be willing to tell this lie.

There is no doubt that Boo Radley would not be found guilty, in a Maycomb county courtroom, of any crime for what he did to Bob Ewell. In fact, it is likely that such a trial would turn Boo Radley into some kind of a hero--something both men want to avoid if they can. They understand that people in town are likely to treat the reclusive man as a hero, something  he just does not have the temperament to endure; too much attention would kill him.

Finally Atticus sees the truth and realizes the sheriff is correct; but now he has to worry about setting the proper model (example) for Scout. She does, indeed, seem to understand what has happened and why they will not be telling the truth about Boo Radley killing Bob Ewell. She understands that Boo is a kind of mockingbird, someone who does nothing to hurt anyone and tries only to help where he is able.