a fish is a kind of aquatic animal & gills are used for breathing water by aquatic animals ->  gills are a part of a fish used for breathing water  water contains dissolved oxygen & water is a part of an environment ->  water is a part of an environment that contains dissolved oxygen  water is a part of an environment that contains dissolved oxygen & breathing is when an animal converts oxygen in their environment into oxygen in blood ->  breathing is when an animal converts dissolved oxygen in water into oxygen in blood  gills are a part of a fish used for breathing water & breathing is when an animal converts dissolved oxygen in water into oxygen in blood ->  breathing is when a gill converts dissolved oxygen in water into oxygen in blood  breathing is when a gill converts dissolved oxygen in water into oxygen in blood & in order to convert oxygen in water into oxygen in blood, fish must take in oxygen from water -> fish use gills to take in oxygen that is dissolved in water