sentences: if a part of one system carries something to a part of another system, then those two systems are interacting
docs: system has parts_interact sub_system is system. organ_system is connected_physical_path_system. exhaust_system is connected_physical_path_system. connected_path_system is path_system. embedded_system is system. navigational_system is system. computational_system is connected_path_system. coordinate_system is system. system is body_part.