The Rule of Animation Conservation (AKA Pencil Mileage) stipulates that, if a work is made through any type of animation, something about the story or the characters has to justify using the medium. This is because animation is relatively expensive. Sure, it looks pretty, and it's fun, but it isn't practical to blow extra time or money to make something animated when it doesn't have to be. Here are some good ways to justify using animation: A specific application of the Law of Conservation of Detail. An animation trope, obviously.