What does Scout say to Atticus that convinces him that Mr. Tate is right?To Kill a Mockingbird by Harper Lee

In Chapter 30 of To Kill a
Mockingbird, Mr. Heck Tate insists that he report that Bob Ewell fell on his
own knife and died, but Atticus Finch believes that Mr. Tate is trying to merely protect
Jem.  He tells the sherriff that he must have Jem live honestly and face the
consequences of his action of stabbing Bob Ewell because he as a father has taught his
children to do so.  However, Mr. Tate insists that Jem could not have exerted the force
with which the knife was thrust into Ewell's "craw":

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"Your boy never stabbed Bob Ewell...didn't come
near a mile of it and now you know it.  All he wanted to do was get him and his sister
safely home."

Then Mr. Tate
tells Atticus that it is not against the law for a citizen to prevent a crime from being
committed.  If this citizen becomes known, all the townspeople would knock on his door
and drag him "with his shy ways into the limelight....to me, that's a
sin."

As Atticus sits on the porch, he finally raises his
head to Scout who has recognized the phrase "that's a sin" from advice that Atticus has
given her.  When Atticus concludes, "Mr. Ewell fell on his knife," he asks Scout if she
understands.  Indeed, she does--thoroughly.  Scout tells her
father,

"Mr.
Tate was right....Well, it'd be sort of like shootin' a mockingbird, wouldn't
it?"

Reminded of his lesson
about the use of bb guns, Atticus is assured that Scout truly does understand.  He is
convinced.