What happened during the fight between Mary and Big Bertha?

After Mary began to excel in school, she was confronted on the playground by the classroom bully, Big Bertha. Big Bertha thought that Mary was purposefully making her little sister look stupid in the classroom. When Big Bertha confronted her, something in Mary snapped and she began punching Big Bertha, arms flailing like a windmill. She received a black eye from the fight, but earned credibility from the rest of the students who tormented her.