This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
Well, why is a raindrop round? Why is lead shot round? Maybe you think shot is moulded like bullets. It isn't. The lead is melted. Then it is showered like rain from a tall shot tower. As it falls the liquid drops turn over and over. A force called gravity pulls all the parts of the little mass toward the center of it. And, as each one whirls the air presses on all sides with equal force. As the drops fall they cool and harden into tiny globes. Once the earth was in a soft, molten state and it, too, whirled through space and around the sun. So it was made round, and it cooled and hardened in that shape. The same forces keep it round. The sun, the moon and the stars are all globes.
If the earth was all water, and if it constantly changed the direction of its whirling as a baseball does when pitched, it should be as round and smooth as a ball. But it is of water, soft earth and hard rocks, so the friction of the air is uneven. And it turns always along one line that runs through the north and south poles as straight as a string through a bead. So along that line the earth is flattened. Midway between the poles where the whirling is most rapid, the earth bulges.