sentences: digests is a mean of physical environment
docs: mammal_digestive_tract is digestive_tract. mammal_digestive_tract is body_part. digestive_system is system. reptile_digestive_tract is digestive_tract. reptile_digestive_tract is body_part. every_living_human_being has digestive_system digestive is substance. digestive_tract is organ_system. digestive_tract is body_part. all_animals has digestive_system