If you are at the center of the earth, gravity is zero because all the mass around you is pulling "up" (every direction there is up!).
(you; are; at the center of the earth) (gravity; is; zero; because all the mass around you is pulling "up" (every direction) (all the mass around you; is pulling; up; every direction there is up!))
you gravity all the mass around you
are is is pulling
at the center of the earth
zero because all the mass around you is pulling "up" (every direction
up every direction there is up!)