Why many things in the universe like planets, stars are round? why not squares or triangles? even the drop of water and substance's molecules are rounds? Why?
Answers
Because of gravity. A planet’s gravity makes things on the planet fall down towards the core of the planet, just like things that fall down into holes tend to stay in those holes because it takes energy to lift it back out, and once everything has fallen as far down as it can, you have a sphere.
For a cloud of gas (stars come from coulds of gas), this is very easy. For a rocky planet, the fact that it is in more or less solid (unless it’s a gas planet) makes it a bit harder.
Small objects in space don’t have enough gravity to collapse themselves into a sphere, this way. For example, one of the two moons of Mars isn’t very big, and it’s shaped like a potato or a peanut, a bit. If it had been bigger, its gravity would have been stronger, and it would’ve turned more spherical.
Being roughly spherical is part of the definition of a “planet”, aswell. So if some large object wasn’t roughly spherical, it wouldn’t be a planet.