Requested anonymously (many times. more times than Nebby escapes the bag.)
It is a truth universally acknowledged that Nebby refuses to stay in the bag. If you’ve played Pokémon Sun or Moon, or seen the recent uprising of Nebby memes, you know this all too well.
This begs the question (or maybe, ‘bags’ the question): How does Nebby keep escaping the bag? Nebby constantly leaves the bag to go on adventures or to the Ancient Ruins, or sometimes just to defy Lillie. Sometimes Nebby is able to do it without Lillie noticing, and sometimes without even having the zipper open. As a Cosmog, Nebby does know the move Teleport, but that looks much different than what it does when leaving the bag:
Nebby seems to pass right through the fabric of the bag. We know Nebby has a gaseous form, since Cosmog is the “Nebula Pokemon”. As we found out in our Cosmog Analysis, Cosmog is most like a molecular cloud, which have a density of about 100 particles per cubic centimeter. For reference, sea-level atmosphere air contains about 2.53×1016 molecules per cubic centimeter (or (0.0012 g/cm3): more than a million billion times as much as a molecular cloud. Using Nebby’s height and weight from the pokedex, we find that Cosmog is even more dense than air: 0.024 g/cm3.
Cosmoem, Nebby’s evolution, is even more dense: 1909.7 g/cm3. If Nebby, a Cosmog, compressed itself into the density of a Cosmoem, Nebby would only have a radius of only 0.23 millilimeters, or roughly 4350 times smaller than it was previously.
Depending on the fabric that the bag is made of, 0.23 mm is most likely small enough to slip between the stitches at the seam, or maybe through the spaces in the zipper.
So what evidence do we have that Nebby can collapse itself into such small sizes? Well, that’s what nebula do. Molecular clouds collapse to create stars, just like Cosmog (the Nebula Pokemon) evolves into Cosmoem (the Protostar Pokemon). Usually, nebulae do this because of gravity: Cosmog is too small to collapse from its own gravity, but we still know it can collapse. Why? Look at its arms: it’s already forming little stars. Cosmog must have the ability to compress itself to form these tiny stars. This ability isn’t unusual for psychic pokémon: Gardevoir, for example, can compress air into black holes. Since Nebby can compress itself, unless the bag is perfectly sealed Nebby will easily escape.