Monsters, Inc.

The parallel city of Monstropolis is inhabited entirely by monsters and is powered by electricity which is generated from the screams of human children. At the Monsters, Inc. factory, skilled individuals called "scarers" access the human world through closet doors in children's bedrooms, to scare the children, and harvest their screams. It is considered dangerous work, as human children are believed to be "toxic" to monsters. Energy production is falling because children are becoming more difficult to scare, and company chairman, Henry J. Waternoose, is determined to find a solution. James P. "Sulley" Sullivan is the organization's top scarer, but is engaged in a fierce rivalry with the chameleon-like monster, Randall Boggs.
One day, Sulley discovers that Randall left a door activated on the scare floor, and a small girl has entered the factory. After desperate failed attempts to put her back, Sulley takes her home. His best friend, Mike Wazowski, is on a date with his girlfriend, Celia, and chaos erupts when the child is discovered. Sulley and Mike escape the Child Detection Agency (CDA) and discover that the little girl is actually not toxic. Sulley grows attached to her and names her "Boo". They smuggle her into the factory in an attempt to send her home. Randall, discovering Boo, tries to kidnap her, but instead kidnaps Mike. Randall reveals that he has built a large machine named "The Scream Extractor" in an attempt to extract all possible screams out of captured human children to help avert the company's production problems. Randall straps Mike to the machine, but Sulley unplugs it and reports Randall to Waternoose. However, Waternoose is secretly in league with Randall and instead exiles Mike and Sulley to the Himalayas. The two are taken in by a Yeti, who tells them about a nearby village which can enable them to return to the factory. Sulley heads out, but a frustrated Mike refuses to follow. Meanwhile, Randall straps Boo to the Scream Extractor, but Sulley saves her by destroying the machine. Waternoose sends Randall to capture Sulley. As Mike returns to reconcile, Sulley temporarily incapacitates Randall and flees with Mike and Boo.
Randall pursues them as they speed through the factory, riding on the doors that are heading into a giant vault where millions of doors are stored. Boo's laughter activates the doors, which allows the pursuit to pass in and out of the human world. Randall attempts to kill Sulley, but Boo attacks him. Sulley and Mike exile Randall into the human world, where two residents at a trailer park beat him with a shovel. Sulley and Mike find Boo's door, but Waternoose sends it back to the Scarefloor. While Mike distracts the CDA, Sulley confronts Waternoose, who reveals that he is working with Randall to kidnap kids and use the Scream Extractor to keep the company from going out of business and to stop its failing energy production. The CDA arrests him after discovering the recorded confession. The CDA's leader is revealed to be Roz, who has worked undercover for two-and-a-half years, trying to expose Waternoose's plot. Sulley and Mike say goodbye to Boo and return her home as her door is subsequently shredded to prevent any more escapes.
With the factory shut down, Sulley is named as the new CEO of Monsters, Inc. He subsequently ends the company's energy crisis by making the monsters enter children's bedrooms to make them laugh, as laughter is ten times more powerful than screams. Mike takes Sulley aside, revealing he has rebuilt Boo's door, and only needs one more piece, which Sulley took as a memento. Sulley enters and happily reunites with Boo.

Question: What activates the doors?
Answer: Boo's laughter