3D animation is digitally modeled and manipulated by an animator. The animator usually starts by creating a 3D polygon mesh to manipulate. A mesh typically includes many vertices that are connected by edges and faces, which give the visual appearance of form to a 3D object or 3D environment. Sometimes, the mesh is given an internal digital skeletal structure called an armature that can be used to control the mesh by weighting the vertices. This process is called rigging and can be used in conjunction with keyframes to create movement. Other techniques can be applied, such as mathematical functions (e.g., gravity, particle simulations), simulated fur or hair, and effects such as fire and water simulations. These techniques fall under the category of 3D dynamics.