Pay It Forward (2000)

A young boy (Haley Joel Osment) is challenged by his teacher (Kevin Spacey) to make the world a better place. The boy comes up with the "pay it forward" concept, in which you do a good deed for someone, who then does a good deed for three other people, rather than "payback" the favor. The boy starts by taking in a homeless junkie, much to the surprise of his mother (Helen Hunt).