5 Answers

For large celestial bodies, roundness is a product of gravity.

When the leftover elements from the sun's creation scattered throughout what is now our solar system, individual particles started attracting each other together with gravity. As more and more particles gathered, they compressed themselves into a shape which allowed them to occupy as small of a space as possible. (due to gravity) In essence, a sphere. Another way of saying this is that the earth's mass reached a point (as it was growing) that its internal gravitational forces overcame any other rigid forces that might have kept the earth in a non-spherical shape.

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It is round because that is the most efficient way to conserve energy as goes on its merry way. Everything in the universe and on planets operates most efficiently. Less is more.

To make it easy to get "AROUND" in 

Actually its not round, its pear shaped.

The fact that it also rotates around its axis helps too...