Suppose you have a huge quantity of matter of any shape, and every part of it is pulled by every other part so that all the material is packed as close as possible. When all the parts are compressed as much as possible and can get no closer, they have taken up the shape of a sphere. No other solid shape has all its parts as close together, on the average, as a sphere does, which is why the Earth, attracting everything to its center, is a sphere.
