In 1989, college senior Wendy Kopp was trying to figure out how to improve public education in the US. For her senior thesis, she proposed creating a national teaching corps that would recruit recent college grads to teach in needy schools. One year later, she launched the nonprofit, Teach for America. Today, TFA has close to 60,000 alumni and continues to place thousands of teachers across the country. PLUS in our postscript "How You Built That," we check back with 19-year-old CEO Abby Kircher who turned a peanut butter obsession into Abby's Better Nut Butter.