A puzzle

[Onkel Dunkel]

By the way; gravity is there because of the mass (of the earth in this case) and the reason why we stay on the surface of the earth is bacause the biggest gravitational force is beneath us (the mass of the earth). If something was in the exact center of the earth, all mass (and thereby gravity) would be around this object, pulling it in every other direction than towards center (a little like if you stand on the magnetic north- or southpole with a compass the it will start spinning around).