Whales are the biggest creatures that have ever lived on the earth. They are even bigger than the biggest dinosaurs (恐龙) were. Scientists think that whales once walked on land. They believe this because whales are not fish—they are mammals (哺乳动物).
Not only are whales the biggest creatures on theearth, they also are some of the smartest. They live in family groups. They communicate with each other, and look for food in groups. Mother whales keep their babies close by, and help them move in the right direction.
Sometimes a whale will lift its head out of the water just to take a look around. Because whales are mammals, they must breathe (呼吸) air. They breathe through a blowhole on their back. However, they can hold their breath for a very long time. Some kinds of whales can hold their breath for two hours at a time. Whales never really sleep. Instead, they swim along with their blowhole above the water.
Many kinds of whales move to warmer waters during the winter months. During the summer, they build up a layer of fat so that during the trip they do not have to eat. Bears also eat during the summer to build up a layer of fat so that they can sleep during the winter. Some scientists think the prehistoric (史前的) whales may have looked like huge bears.
