In chapter 15, Atticus sits outside of Tom Robinson's jail cell as the Old Sarum bunch arrives from the northern part of the county to lynch Tom before the trail. Jem and the children follow their father to the Maycomb jailhouse and watch as the Cunningham family surrounds Atticus. Scout, who does not understand the gravity of the situation, becomes curious and runs out into the middle of the group of men. Scout becomes embarrassed and begins to search for a familiar face.

Fortunately, Scout recognizes Mr. Cunningham, Walter Jr.'s father, and mentions that she goes to school with his son. However, Walter Cunningham refuses to respond to Scout, who begins to talk about his entailment. Eventually, Mr. Cunningham is able to view the situation from Atticus's perspective and finally acknowledges Scout. Mr. Cunningham realizes that Atticus is in a precarious situation and sympathizes with him. Walter Cunningham then tells the Old Sarum bunch to leave and the mob disperses. If it wasn't for Scout's presence, Atticus may have not been able to prevent the mob from lynching Tom Robinson. 

As Atticus explains to Jem on the morning after they had dispersed the lynch mob,

"A mob's always made up of people, no matter what. Mr. Cunningham was part of a mob last night, but he was still a man. Every mob in every little Southern town is always made up of people you know--"

Atticus's presence alone would probably not have been enough to prevent the group from taking Tom from the jail and murdering him (the unseen Mr. Underwood and his shotgun standing guard above may have made a difference, however), but the appearance of the innocent young children did make a difference. Scout's simple conversation with Mr. Cunningham--about his entailment problems and about his son, Walter Jr.--shamed Cunningham enough to order his friends away. Perhaps they thought better of their intentions, or maybe they just weren't willing to take such murderous actions in front of the children, but Scout's words were able to break down the barriers between a mob mentality and individual human thought. Cunningham--the man--separated himself from the mob and made a personal decision to leave justice in the hands of the jury.