Pixar Animation Studios has been hiding Easter eggs connecting the greater world these films reside in for years. Some of the most well-known include the A113 code seen in each entry and characters like Nemo from Finding Nemo appearing in Monsters, Inc. But one Easter egg in Monsters, Inc. has often been missed by fans since its release in 2001, and that's Boo's real name.

In the film, Boo is a toddler who walks through her closet door at night into the world of Monstropolis, more specifically, Monsters, Inc. Children and other human paraphernalia are believed to be toxic to monsters, but that is quickly disproven when the young girl is found latched to the film's hero, James P. Sullivan. Because she is a toddler, who doesn't know many words, and because neither Sulley nor his roommate, Mike Wazowski, know her name, they call her Boo, a word she often employs when jumping out to scare them.

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Boo is a handful at first, and both Sulley and Mike are unsure of how to take care of her and return her to her world without getting in trouble. As they formulate a plan, Boo draws pictures until she begins to grow tired. Sulley takes her to his room, but rather than sleep on the floor as Sulley had planned, she claims his bed. But Boo has issues sleeping because she is scared of monsters in her closet, specifically Randall, the film's antagonist. When she takes out her drawing of him, viewers can get a glimpse of another paper with her name written in the corner: Mary.

Canonically, Boo is named after her voice actor, Mary Gibbs. Gibbs was also a toddler at the time of filming, which gave the film and her character more heart, as well as a sense of realism to her voice. The casting decision helped viewers develop an even deeper connection with the character, thanks to her voice and the toddler's movements. It brought an innocence to Boo that made her adorable to adults and relatable to younger children.

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Since Monsters, Inc. was released in theaters, Pixar has gained even greater mastery with its Easter eggs. Some connections include an Incredibles poster in Coco and the scattered appearances of B&L, the massive conglomerate that contributes to the end of the world in Wall-E. While many of these teases have been expertly hidden in plain sight, they have never been as elusive as Boo's real name.

Throughout the events of Monsters, Inc., Boo's name is never mentioned aloud. However, since Sulley was able to reunite with her at the end of the film, it's possible that he finally got to learn of her real name after visiting her again. Out of Pixar's many hidden details, the brief moment showing Boo's name is still one of the most clever reveals in the studio's history.

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