What challenging task does Heck Tate ask Atticus to help him with, to the children's surprise?

In chapter 10, Jem spots a rabid dog staggering down the main street of Maycomb and tells Calpurnia who immediately informs the neighbors that there is a dangerous dog on the loose. Shortly after Calpurnia knocks on the Radley door to warn Nathan that a "mad dog’s comin‘," Sherrif Tate arrives at the scene with Atticus. After surveying the situation, Sheriff Tate hands Atticus his rifle and asks Atticus to shoot the rabid dog. Sheriff Tate tells Atticus that he cannot shoot well and is aware that Atticus is an expert marksman. Atticus reluctantly takes Sheriff Tate's rifle and kills the rabid dog in one shot. Both Jem and Scout are astonished at their father's marksmanship and never knew that Atticus had the most accurate shot in the county. After initially lamenting about Atticus's lack of talent earlier in the chapter, Jem and Scout are proud of their father's unique talent after witnessing him kill the rabid dog at Sherrif Tate's request. Following Atticus's shot, Miss Maudie informs the children that their father's nickname as a young man was Ol‘ One-Shot.