Purely History on Instagram: "Mary Gibbs, who voiced Boo in Monsters, Inc. (2001), was only about two years old during production. She was too young to sit in a recording booth and read lines from a script, so the Pixar team recorded her as she played around the studio. They captured her natural giggles, babbling, and reactions, then pieced together those sounds to create Boo’s dialogue in the film."