In Pixar’s Monsters, Inc., monsters travel into children’s rooms through doors.
The company stores thousands of doors in a vast warehouse and sends scarers through them to reach children’s bedrooms. Each door acts as a connection between the monster world and a particular child’s room. The door system is central to the film because Boo follows Sulley back through one and becomes lost in Monstropolis.
These doors are not ordinary entrances in the story’s fantasy rules. They are handled on a large industrial track, identified and stored by the company, and treated as valuable equipment. The film also distinguishes ordinary door travel from the machine used to destroy doors, which is a separate device. That distinction matters because Sulley later tries to protect Boo’s door rather than destroy it.