Almost all animals need to sleep to stay healthy. But sleeping can also be dangerous because other animals may hurt those who are sleeping. This leads to many different ways of sleeping that help them stay safe. Some of these ways may surprise you!

    Almost all bats sleep upside down. They hang from a branch of a tree or the ceiling of a cave. Up high, bats are safe from most other animals. If they are in danger, they just fly away.

    A dolphin sleeps with just half its brain at a time. It must stay awake to breathe and to watch for danger. Dolphin mothers and babies can go without sleep for as long as two months to stay safe.

    Some ducks sleep with just half their brains, too. These ducks often sleep in a line. The ducks at both ends help to keep the group safe. They keep one eye open to watch for danger. The ducks in the middle sleep deeply with both eyes closed.

    Seals breathe air, but that doesn't stop some of them from sleeping underwater. They hold their breath and dive deep to sleep for up to twenty minutes at a time. Sleeping deep underwater helps these seals stay safe from animals that hunt them near the surface.

    Horses often sleep standing up. Their legs lock in place so they don't fall over. If a horse is lying down, it needs a long time to get up. A horse can run from danger more quickly if it is already standing.

    A flamingo can sleep while standing on one leg. The standing leg locks so it stays straight. The other leg remains close to the flamingo's body. Its head rests on its back. A flamingo lives (and sleeps) near many other birds to stay safe.

    Some seabirds can sleep while they fly. These large birds spend most of their lives over the ocean. They are in danger on the water, but they are safe in the air. They fly for many hours without flapping（拍打）their wings. Sometimes they sleep while they are flying.

    Would you like to sleep how they sleep?

    Winter is very cold in many countries. It's easy for you and me to keep warm. If we are cold, we can just buy a warm coat or use a heater and we will feel better. But what about animals? How do they keep warm in winter?

    Some animals migrate(迁移). This means they travel to places where the weather is warmer. Many birds leave before the winter starts. They fly in groups. If they fly together, the trip will be safer. They like to migrate to the same places every year. Do you know how they can find the same places? Scientists say that birds use the sun, moon and stars to help them. Other animals like moose (麋鹿) also migrate. Fish and whales swim to warmer water.

    Some animals don't have leave their homes. They hibernate in winter. They fall into a very deep sleep. The animals' temperature drops and their breathing slows down. Before the winter, these animals must eat too much food that they get very fat. If they don't eat enough, they may die when they are hibernating.

    When we talk about the universe, we mean not only the earth, the sun and the moon, but also the stars and space between them. There are millions of stars in our sky. The sun is one of them. Around it there are eight big planets moving, including the earth. The moon is our nearest neighbor in space. If you look at the sky on a clear night, it is possible to see about 3, 000 stars. They look small, but they are really very big hot balls of burning（燃烧着的）gas. Some of them arc huge（巨大的）, but others are much smaller like our planet Earth. The biggest stars are very bright, but they can only live for a short time. Every day new stars are born and old stars die.

    All the stars are very far away. The light from the nearest star（except the sun）takes more than four years to reach the earth.

    Hundreds of years ago, people used stars, like the North Star, to know which direction（方向）to travel in.

Today you can still see that star.