It all started with the social studies teacher's extra-credit assignment: come up with a plan to change the world for the better, and do it. Twelve-year-old Trevor McKinney began by doing something good for three people. But instead of paying him back, he asked them to "pay it forward" by doing a favor for three more people, who in turn would help three others, and so on, each act a link in a chain of human kindness. And no one -- not his teacher, his mom, or anyone in his small California town -- could ever have dreamed of how far Trevor's plan would go.