system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do planets and extraterrestrial bodies always appear as a spherical shape? I.e. why not square?
gpt
LARGELY sphere-LIKE, not necessarily spherical. Some bodies rotate so fast that they're actually thicker along their "middle" and thinner from pole to pole. Others have big craters and stuff that mar their surface. And a great great many comets, asteroids and small moons aren't even close to spherical. Phobos and Deimos, the moons of Mars, look like potatoes.

But the big ones are generally sphere-like because that's how gravity works. It pretty much evenly pulls everything toward the centre (with lighter stuff like air being pulled less and metals being pulled more).

So if you get enough mass into a planet-sized object, the inward force of gravity will have a tendency to pull down the taller bits and fill in the lower bits until everything is an even distance away from the centre. So taller bits will spread out to the sides and lower bits will get filled up by the stuff that falls down the taller bits. Gravity attracts uneven stuff downward, and the more gravity (caused by the denser or larger the planet), the bigger the pull.

This doesn't work so well with big chunks of hard rock - and that's why we have mountains and why old, cold small moons or comets or asteroids that were once maybe a part of a planet are irregular - but it works great with gasses and liquids, and works quite well with dust and sand. 

Considering most of the larger planetary bodies were either liquid or gas or dust at some point in their past life, gravity would have shaped them more-or-less into a sphere as their surfaces flowed around.