Observation 1: Physical Development. In order for our brain to make room for new pathways of communication and keep what is used most running smoothly, unused neurons need to be pruned. At one point in the movie, workers in Riley's mind are going through all of her memories, dumping unnecessary ones, such as all the names of the presidents aside from “...Abraham Lincoln, George Washington, and the fat one...” This is Pixar's comedic take on a necessary developmental process.
I totally can't believe how many people loved this tree trimmed like this. Because in CA. anyone who loves gardening does this a LOT to their trees here. I guess it's not all that common everywhere else?