sentences: gravity causes objects that have mass / substances to be pulled down / to fall on a planet
docs: solar_gravity is gravity. gravity has effect_of_pulling_things_down gravity is force. everything_with_mass has gravity gravity is force_in_universe. gravity is what_keeps_things_on_earth. heavier_objects has greater_gravity_than_small_objects gravity is property_of_matter. gravity is attractive_force_felt_between_objects. earth has gravity