I'm assuming we're talking about the planet earth, and not say, oh, a piece of candy. Bringing it all down to it's simplest explanation, under the forces of temperature, pressure, and the addition of gravity (defined as the force that tries to pull all things to the center of the earth), solids (like metals,such as nickel) condense into a ball, and more liquid or viscous substances like magma and water, get pushed aside, to the outer perimeter. The continuous bombardment of gravity from all sides makes it into a ball. The resulting object with a nickel and heavy metal core, and a viscous magma and water outer shell, is called a planet.
