It all started with the social studies teacher's extra-credit project: Think of an idea for world change, and put it into action.

Whilst this proved a little ambitious for most of his classmates, twelve-year-old Trevor thought he would start by doing something good for three people. But instead of paying him back, he would ask them to "pay it forward" by doing a favour for three more people.

If it all went to plan, Trevor thought, it would be the start of a long chain of human kindness . . .

Sound unlikely to you? Well a lot of other people had their doubts too - Trevor's teacher, his classmates, his mother, in fact everyone in his small California town.

It could never really work. . . could it?