system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are galaxies round as a pizza instead of round like a sphere?
gpt
Ever watched one of the many videos that the astronauts on the ISS send back to earth? They're fascinating! Many videos involve water (or other liquids) and how it behaves in free fall. Put a bunch of water together, just floating, and it will stay a sphere. 

If you spin it, however, it flattens out like a pizza! Centrifugal force from the spin causes the molecules to try to escape, but water's tendency to stick to itself holds it together at low speed and makes the water go oval. If you did the same thing with a more sticky substance (like pizza dough) it could go faster and flatten out even more before breaking apart. 

You could have a spinning sphere if the water was able to spin in every direction at once, but since the molecules bump into each other, they all cancel each other's movement out until they're all moving in the same direction at the same speed. That's why galaxies mostly all spin like pizza dough.