Why do Disney and Pixar animated movies contain the 'A-113' easter egg?

I have noticed that many Disney and Pixar animated movies contain 'A-133' in some (random) places.

For instance:

Toy Story:

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A Bug's Life:

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Cars:

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Brave:

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Monster's University:

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Is there any explanation to this?

disney pixar

Best Answer

According to Wiki:

A113 (sometimes A-113 or A1-13) is an inside joke, an Easter egg in animated films created by alumni of California Institute of the Arts, referring to the classroom used by graphic design and character animation students including John Lasseter and Brad Bird.

You can see John Lasseter explaining it here:

Here are some of Pixar's main figures and the famous classroom:

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EDIT: The Wiki page above offers a pretty comprehensive list which proves that the A113 easter egg has spread to many other places, including a huge number of animated shows as well as live action films like The Hunger Games: Catching Fire and Mission: Impossible 4. Here's a blog page that displays some of those non-Pixar examples.