An important element of "Inside Out" is the idea of memories, and how crucial they are to our emotions. In the story, the emotions inside Riley's head are able to conjure memories at any time in the form of little glowing balls which can then be relived in detail. Every memory has associated emotions, color-coded with, for example, yellow for joy or blue for sadness, and several moments in Riley's life are seen through this narrative device. What's particularly strange is that in some of these memories, other Pixar movies can be seen.
While this may be more of a Pixar Easter egg than anything, it's still an interesting creative choice to place the studio's previous work into a library of treasured memories. In particular, throughout the halls of memory cores can be seen glimpses of the opening from "Up" in Riley's memory banks, which implies that she may have seen the movie in her lifetime. This would imply that some Pixar movies exist as movies within the universe of other Pixar movies, which would be pretty trippy. This also further complicates the elaborate "Pixar Theory" that has gained steam online, which posts that all the Pixar movies actually take place in the same universe, including apparent outliers like "Cars" and "Brave."
