Jem's growth spurts prior to Chapter 14 of To Kill a Mockingbird had created a gap between the two children that angered Scout. His
... maddening superiority was unbearable these days.
Jem had scolded her for antagonizing Aunt Alexandra, and he ordered her to stop. When he compared himself to "grown folks" and threatened to "spank" Scout, she could take no more. Recognizing that he would be an easy target since he was sitting on the bed,
... it was easy to grab his front hair and land one on his mouth.
Calling him a "damn Morphodite," Scout attacked, landing the first punch. Jem soon overpowered her, and it took Atticus to split them up, but Scout was happy in defeat because, once again, the two children
... were still equals.
When Jem fights with Scout in Chapter 14, she is satisfied that their normal relationship is restored.
As Jem matures, he begins to distance himself from Scout. Jem also understands adult situations better now. He can often detect people's insinuations, as well as the different shades of meaning conveyed in discussions of the circumstances that relate to the upcoming trial. Because of this new understanding, Jem attempts to explain to Scout that she should try not to "antagonize Aunty" and not worry their father because he has many things on his mind:
I said Atticus didn't worry about anything....
"That's because you can't hold something in your mind but a little while....It's different with grown folks, we--"
When she hears Jem's allusion to himself as "grown folks," accompanied by his insult to her intelligence, Scout comments, "His maddening superiority was unbearable these days." Further, in a continuation of his attitude of superiority, whenever Jem reads something, he passes it along to Scout as has been his habit; however, now it is given to her, not so much for her enjoyment, but for her "edification and instruction."
Angered and offended by his scoldings, Scout asks Jem, "Who do you think you are?" Jem tells her he will spank her, and Scout becomes enraged. She grabs a lock of his hair above his forehead and punches him in the mouth with her fist. Jem slaps her; Scout strikes him again. Enraged, Jem punches her in the stomach, sending her to the floor. Scout remarks:
It nearly knocked the breath out of me, but it didn't matter because I knew he was fighting, he was fighting me back. We were still equals.
Hearing the scuffle between Jem and Scout, their father intervenes, sending both of them to bed. Scout is delighted because Jem is sent to his room at her bedtime, rather than at his later hour. As she hears Jem call out, "Night, Scout," she knows that their old relationship is, indeed, re-established.