In the movie Pay It Forward, a schoolteacher assigns his class the task of finding a way to bring about positive change in the world. Trevor, one of the students, decides to do three good deeds for others—three acts of random kindness—and in return ask them to do three good deeds for three other people, who will be asked in return to do the same for others, and so on. If every person who has just been helped pays this help forward to three others, then within twenty-one rounds, everyone on earth would have been helped