Whale is an aquatic animal and it looks like the fish in several respects.. Hence, it is normally considered as a fish. But actually it is a mammal. It is developed from the giant reptiles known as ‘dinosaurs’. The blue whale is the biggest animal. Its length is up to 30 meters and its weight is about 125 tons. The tail of blue whale is 6 meters wide. The weight of its bones is 20 tons and its muscles weight is 50 tons, whereas the weight of its tongue is about 2 ½ tons. Now there is a question that why is the whale considered as a mammal and not a fish. Whale does not have gills to breathe like other fishes whereas it has lungs to breathe like man. It has a nose which is situated in the front part of its head. When its nose gets closed in water, it brings its head above the surface of water after every five to ten minutes for breathing. It first breathe out the air from its body, producing a loud voice capable of being heard from long distances and then it breathe in air and goes inside the water. It can stay for about 45 minutes in water without breathing in air. It can go up to 60 meters deep in water. As a mammal it gives birth to young ones and feed them and keeps them with itself. It has a heart like man, As it is warm blooded creature, therefore it can adjust its body to the environment of water. As the most parts of its body look like those of mammals therefore whale is considered as a mammal not a fish.