How did Jem break his arm?

In the first sentence of the novel, Scout says that Jem broke his arm. She starts to explain what happened but says that she needs to go back and provide the necessary context in order for the story to make sense. The rest of the novel is the background context for Jem’s broken arm. At the end of the novel Bob Ewell, who has suffered as a result of Atticus’s defense of Tom Robinson, attacks Jem and Scout on their way home from the Halloween pageant. Jem breaks his arm in the struggle. The story of a broken arm serves as a narrative device, bookending the entire novel with Scout’s telling of the story. While initially the reader might assume Jem broke his arm through innocent childhood games, by the end of the novel we understand the darker, more complicated truth behind the accident.