What crisis shows the children a surprising skill their father possesses in To Kill a Mockingbird?

At the beginning of Chapter 10, Scout describes her father
as "feeble," and creates a list of things that Atticus is incapable of doing. His
advanced age and bad eyesight are just two of his inadequacies that seem to embarrass
Scout. But when a mad dog is seen stumbling down the street, who does Sheriff Heck Tate
hand his rifle to in order to dispose of the animal? None other than
Atticus.

    
With movements so swift they seemed simultaneous, Atticus' hand yanked a ball-tipped
lever as he brought the gun to his shoulder.
     The rifle cracked. Tim
Johnson leaped, flopped over and crumpled on the
sidewalk...

The children
discovered a talent that Atticus possessed but had never discussed with them: that of
the crack marksman. In his youth, Atticus had been known as "Ol' One Shot" Finch, the
best shooter in the county.