sentences: some animals' gases grows thicker for staying warm in the summer
docs: bird is warm_blooded_animal. grey_red_backed_vole is mammal. mammal is warm_blooded_animal. weta is animal. humans is warm_blooded_animals. mammals has warm_blood helium_gas is heavier_than_hydrogen_gas. warm_blooded_animal is animal. rabbits_and_birds is both_warm_blooded_animals. animal is aerobic_organism.