How and why are planets spherical? What makes them round?

Planets are spherical due to the force of gravity. Gravity pulls all matter towards the center of mass, which causes an object to become more and more compressed towards its center. This means that as a planet forms and becomes more massive, its own gravity causes it to become round. This is because any unevenness in the planet's shape would result in the gravitational force being stronger in some places than others, which would cause the less massive areas to be pulled towards the more massive ones. Over time, this gravitational force causes the planet to become more and more spherical, as this shape distributes the planet's mass equally in all directions. Additionally, a spherical shape allows for the most efficient use of space, meaning that a planet with a spherical shape can hold the maximum amount of material in the smallest possible space.