The dolphin is a member of a group of sea animals closely related to whales and porpoises. The dophin is a mammal and not a fish. Dolphins are ranked as one of the most intelligent animals, and they also have highly developed hearing. They live in either saltwater, like oceans, or freshwater, like rivers. Dolphins range from 4 to 30 feet, and they can weigh from 100 pounds to 10 tons. Dolphins have a torpedo-shaped body, which helps move them through the water quickly. They breathe through a blowhole, which is a nostril on the top of their head.