POPULAR SCIENCE

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Plants give us oxygen. If all plants die，you will die too. Without plants you can?蒺t breathe. But you also need energy. You need it to breathe and to eat and to move. In fact，you need energy to live.

Some of the early animals can?蒺t make their own energy. They need the energy of sunlight，but they can?蒺t make it themselves. How can they get it？There is only one answer—eat the plants.

That is still true today. Animals still have to get their energy from plants. Plants keep you alive. Sometimes we eat the plants directly. But sometimes an animal eats the plants first，then we eat the animal.

Apples and oranges grow on tree-plants. Bread comes from plants in a field. We get eggs from birds，but the birds eat plants. Or they eat insects，and the insects have eaten plants.

We can eat meat from a deer，but the deer has eaten plants. We eat fish and the fish has already eaten plants. Or it ate other fish，and they ate plants. We don?蒺t eat grass，but we drink milk. And the cow has eaten the grass for us.

Every plant of your food comes from plants. When you eat part of an animal，ask yourself，“What did this animal eat？”If it ate other animals，ask yourself，“What did they eat？”You will always reach a plant.

So what is really keeping you alive？The green plants of the world are catching sunlight for you. You are using the energy from our own star.

Water is found almost everywhere. Even in the driest part of the world there is some water in the air. You cannot see it or feel it when it is part of the air. The water in seas and lakes and rivers is liquid. The water in the air is not liquid but gas. We call it water vapour（蒸汽）.

Clouds are made of water. They may be made of little drops of water. They may also be made of snow crystals（晶体、结晶）. Snow crystals are little crystals of ice. Ice is frozen water or water that has become solid.

Water，you have found out，may be solid，or liquid or gas. When it is solid，it may be as hard as rock. When it is liquid，you can pour（倒） it out of a container（容器）. When it is gas，you cannot see it or feel it.

The universe means not only the earth，the moon and the stars，but also all the things too far away to see in space. Scientists have found out a lot of “new” stars in the past fifty years.

We see that the stars in the sky are smaller than our earth. But many of them are even bigger than the sun. They look smaller than the sun because they are much farther away from the earth.

The moon is the satellite（卫星） of the earth. It is our nearest neighbour in space.

With the help of man-made satellites，scientists can do much research in many fields of science.