Trivia: George, the monster from Monsters, Inc. who gets the sock on his back, makes an appearance in this film as a member of the team who gets disqualified in the first event of the scare games.
Trivia: When Mike runs over a student with his floor waxer, the student yells a Wilhelm scream.
Trivia: At the end of the movie, Mike and Sulley begin their career at Monsters Inc by working in the Mail Room. Also working in the Mail Room is The Abominable Snowman we met in the original (who is voiced by John Ratzenberger). This is making a reference to the TV show Cheers, where John Ratzenberger also played a mailman.
Answer: Well a door first needs to be activated from the monster side. See the doors of the human world lack the receiver for the energy they need to open the doorway between worlds (the red light on top). That's why in the monster world they hook a door up to a machine to activate it allowing the passage between worlds before any scream or laugh is made. It costs power to activate a door to allow a monster in. Boo was able to activate doors because she was in the monster world and her screams and laughs triggered the devices on top of the doors, activating them automatically. This was the first time ever a human entered the monster world.
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