Introduction

Dolphins are the most amazing creatures on Earth. Like fish, they spend all their lives in the sea. But unlike fish, they breathe air, have warm blood and suckle their young. They are more similar to humans than fish because they are mammals. Dolphins are mostly about the same size as humans. Porpoises are also about the same size, although a bit smaller. Dolphins belong to a group of whales called Cetacea. The word comes from the Greek word 'cetus' meaning sea monster. Because dolphins are mammals, they breathe air. Dolphins breathe through a blowhole on top of their head as they rise to the surface. Dolphins usually breathe several times a minute, although they can hold their breath for five minutes or more when diving.