The first step in creating a computer generated movie is to create the characters in the story and the world they live in. Each of these objects is modelled as a surface made up of connected polygons (usually triangles). The vertices of each triangle are stored in the computer memory. It's important to know which side of the triangle is on the outside of the object or character. This information is encoded in the order the vertices are stored in, according to a right-handed screw rule: curl the fingers of your right hand around the triangle in the order given by the vertices. There's only one way to do this and your thumb will end up on one side of the triangle — that side is the outside. If you try this with an example, you will find that the outward direction (called the outward normal) of the triangle (A,B,C) is in the opposite direction to that of (A,C,B).