The Earth is round because of gravity. Gravity pulls equally from all sides, so the Earth's mass is evenly distributed. This makes the Earth a sphere, which is a three-dimensional circle.

If the Earth were not round, its gravity would pull more strongly on some sides than on others. This would cause the Earth to become more elongated, like a cigar. However, the Earth's gravity is strong enough to keep it spherical.

The Earth's rotation also contributes to its roundness. As the Earth rotates, the centrifugal force pushes outward on the Earth's surface. This outward force is strongest at the equator, and it helps to make the Earth bulge out there.

The Earth is not perfectly round, however. It is slightly flattened at the poles and bulges out at the equator. This is because the centrifugal force is strongest at the equator, and it counteracts the force of gravity.

The Earth's shape is also affected by its tectonic plates. The tectonic plates are constantly moving, and they can cause the Earth's surface to bulge or sink. This can also cause the Earth's shape to change slightly over time.

Overall, the Earth's shape is a result of the forces of gravity and centrifugal force. These forces work together to keep the Earth spherical, but they also cause the Earth's shape to be slightly irregular.