There’s long been a theory circulating the Internets that all of the Pixar moves exist in the same universe and are all connected. You’ve probably known about some of the hints for a while–A113 keeps showing up, there’s a reference to Monsters Inc in Brave, but some of these I completely missed.
Now, Disney CONFIRMED a bunch of the sightings and added some more you might never have known about (Riley from Inside Out showed up…where?!?!). Here’s the video they released on the official Toy Story page showing how all the movies are connected.
If you want to dive REALLY deep, there are sites dedicated to the chronological order of the Pixar Universe and how the movies actually work together as one long story, instead of just a bunch of Easter Eggs.