The Door is a major way of going from room to room in the Monsters, Inc. factory from the movie of the same name. Scarers use this technology to enter the Human World and scare children.
If a door led to the bedroom of a child that is unable to be scared, then it is declared "dead" and is immediately destroyed using a shredder, leaving only the doorknobs intact (although it is revealed at the end of the first film that the door's remains can still be put back together and therefore repairing it instantly). Doors can also be destroyed if dropped from a great height (such as the one Randall dropped from a conveyor belt in the door vault, prompting Mike and Sulley to enter it immediately before it is smashed, and the door leading to the Trailer Son and Mom's trailer and hence Randall's defeat) or overloaded with an unusually strong scream (such as the door leading to the cabin as a result of Mike and Sulley scaring several human adults).
In Monsters University, due to it being a prequel, all of the doors' stations were much larger, more rectangular, and clunkier in appearance, probably so that they all appear very old and outdated compared to the original film. It should also be noted that all of the doors in both films are made of wood, except for the one used for banishing monsters to places in the Human World like the Himalayas, which is made of metal.
Pixar created all of the doors in the door vault from combinations of twenty-six paint colors, twelve styles, eight wood colors, ten decals, six door knobs and three hardware types. This can create a maximum total of 449,280 possible combinations.
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