When open, doors admit people, animals, ventilation or light. The door is used to control the physical atmosphere within a space by enclosing the air drafts, so that interiors may be more effectively heated or cooled. Doors are significant in preventing the spread of fire.
They also act as a barrier to noise. Many doors are equipped with locking mechanisms to allow entrance to certain people and keep out others. As a form of courtesy and civility, people often knock before opening a door and entering a room.