human
Why is the Earth a sphere?
assistant
The Earth is a sphere because it is a planet. It is a planet because it is in orbit around the Sun. The Sun is a star, and stars are spherical because they are made of gas. Gas is spherical because it is made of atoms. Atoms are spherical because they are made of protons and neutrons. Protons and neutrons are spherical because they are made of quarks. Quarks are spherical because they are made of gluons.