“TerraKids” is the name of a Web site that is, for the present, purely fictional. It is a safe, secure site where 8- to 12-year-old kids become catalysts for a greener Earth by learning about the environment and the role they and their families can play in reducing global warming. Today, TerraKids exists only in thought and in this essay, but it could be live on the Web in the very near future. By making use of what we know about a science of behavior change, we can build a Web site that can help move millions of people to a greener world.