The children were getting very bored with summer, until they thought of things to do that evoked the fear of Boo Radley. The little group was going to diverge along gender lines over this issue. The boys wanted to experience the thrill of learning about the unknowns of Boo Radley, while Scout, by every right remained an innocent girl not so sure about the plans. When she attacked Jem's character he got mad and made sure he got back at her by pushing too hard in the tire.
An even bigger change in their demeanor comes along when they decide to play the Boo Radley Game. They generally craft storylines well with Dill being a villain, and Jem a hero while Scout would get whatever was left. Jem was Boo in this game, but the kids eventually get in trouble because Atticus could tell something wasn't right. As they just about get caught (besides just being Scout's hero by going to the Radleys' to get the tire, or by being the hero in the storyline for a drama) Jem saves the day by dealing with Atticus as the representative of the three.