例句：That did it. Here was a kid who had flunked two grades, who had been told he was‘brain-damaged，’who had been called‘Frankenstein’by his classmates and told his brains must have leaked out of the cut on his head. Suddenly he discovered he could really learn and accomplish things. The result? From the last quarter of the eighth grade all the way through high school, he never failed to make the honor roll；in high school he was elected to the national honor society. Once he found learning was easy, his whole life changedThat did it. Here was a kid who had flunked two grades, who had been told he was‘brain-damaged，’who had been called‘Frankenstein’by his classmates and told his brains must have leaked out of the cut on his head. Suddenly he discovered he could really learn and accomplish things. The result? From the last quarter of the eighth grade all the way through high school, he never failed to make the honor roll；in high school he was elected to the national honor society. Once he found learning was easy, his whole life changed.来自《人性的弱点》

请问这里的that did it什么意思？