RE: Why is the earth round?
The Earth (as well as all the other planets) appears round due to the phenomenon called 'hydrostatic equilibrium.' At a sufficient mass, the gravitational force causes a planet to shape itself into a sphere, as this shape minimizes the potential energy. This is the same principle seen when water droplets form spheres in zero gravity. It's a misconception that other planets are flat - they generally follow the same spherical principle due to gravity.