A building's shell is the component that divides the structure's inner space from its outside and surrounding locations. It may include a variety of characteristics, the most typical of which are the foundation, roofing, doors, windows, and footers. The shell of a building is responsible for preventing the elements from reaching the interior of the structure while at the same time providing visibility from the outside in case of emergency or security reasons. The shell of a building consists of three primary components: the foundation, the walls, and the roof. The foundation is the layer of rock or other material on which the building rests. The walls are the external surfaces of the building that contain its rooms. The roof is the covering over the walls and floors of a building. There are several types of roofs including flat, sloped, gable, and tent. Buildings are usually constructed with concrete or steel frames containing glass, plastic, or other materials used to create the exterior surface. The shells of buildings are typically divided into two main categories based on how they are constructed: load-bearing and non-load-bearing. Load-bearing shells support the weight of those inside the building without assistance from any other part of the structure. These types of shells are typically made out of concrete or steel and are called "bones" of the building because they provide structural support for the building within.