“Scout,” he said, “Mr. Ewell fell on his knife. Can you possibly understand?”
Atticus looked like he needed cheering up. I ran to him and hugged him and kissed him with all my might. “Yes sir, I understand,” I reassured him. “Mr. Tate was right.”
Atticus disengaged himself and looked at me. “What do you mean?”
“Well, it’d sort of be like shootin’ a mockingbird, wouldn’t it?”
He turned out the light and went into Jem’s room. He would be there all night, and he would be there when Jem waked up in the morning.

– Harper Lee

To Kill a Mockingbird, Chapter 30. Atticus asks Scout if she understands why they are keeping Boo Radley’s involvement in the Bob Ewell’s death a secret. Scout understands that Boo, who saved her and Jem, was like a mockingbird and they had to protect him.