Why is J.R.R. Tolkien's Lord of the Rings trilogy so compelling? How about The Matrix or Harry Potter? What makes these disparate worlds come alive are clear, consistent rules for how people, societies -- and even the laws of physics -- function in these fictional universes. Author Kate Messner offers a few tricks for you, too, to create a world worth exploring in your own words. Lesson by Kate Messner, animation by Avi Ofer.