Atticus shoots the mad dog in chapter 10 and surprises his children, who were under the impression that he was a talentless, boring person. Before the drama unfolds, Jem and Scout lament that Atticus has no special talent and is too old to do anything interesting or fun. Later on, the children spot a rabid dog walking down the street and Calpurnia notifies the authorities. Sheriff Tate and Atticus arrive on the scene, and the sheriff looks to Atticus for help. Because of their negative perception of Atticus, Jem and Scout are astonished when Sheriff Tate hands Atticus his rifle, and Atticus kills the rabid dog in one shot. Scout describes the dramatic moment by saying,
With movements so swift they seemed simultaneous, Atticus’s hand yanked a balltipped lever as he brought the gun to his shoulder. The rifle cracked. Tim Johnson leaped, flopped over and crumpled on the sidewalk in a brown-and-white heap. He didn’t know what hit him (Lee, 88).
After killing the rabid dog, Atticus picks up his glasses, and Jem is paralyzed by the spectacle. The Finch children find it hard to comprehend what they witnessed, and Jem is awestruck by his father's extraordinary talent. Once Atticus leaves the scene with Sheriff Tate, Miss Maudie provides insight into Atticus's outlook on life and describes his hidden talent by telling the children,
If your father’s anything, he’s civilized in his heart. Marksmanship’s a gift of God, a talent—oh, you have to practice to make it perfect, but shootin’s different from playing the piano or the like. I think maybe he put his gun down when he realized that God had given him an unfair advantage over most living things. I guess he decided he wouldn’t shoot till he had to, and he had to today (Lee, 90).
When Scout wonders why Atticus is not proud of his exemplary shooting abilities, Miss Maudie replies, "People in their right minds never take pride in their talents" (Lee, 90). Following the incident, Scout declares that she cannot wait to brag about her father's special talent at school, but Jem encourages her to be humble by not saying anything about it.
It should be noted that different editions of To Kill a Mockingbird will have these quotes on different pages, but if you look for them in chapter 10 you should be able to find them.