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 favour for three more people, who in turn would help three others, and so on. No one in his small California town could ever have dreamed of how far Trevor's plan would go.