Human: Why is the Earth round? Assistant: 
The Earth is round because it is a planet. The Earth is not a perfect sphere, however, so it is actually slightly pear shaped. The reason why the Earth is not perfectly round is because the Earth has a slight bulge in the middle. This is caused by the fact that the Earth is made up of a molten iron core that is surrounded by a rocky mantle. The iron core is denser than the mantle, so it is pushed into the center of the Earth.