A smile will tell people around you that you are a kind and friendly person. However,many people don't smile because they think they ugly teeth. For example, one of my classmates, Mile, doesn't have very nice teeth,so thatis why he seldom opens his mouth.he even says that he has lost cofidence because of his teeth.So it is very important for us to look after our teeth. In fact, a lot of people have this problem. Some people inherit teeth problems from their parents,while others'teeth are damaged because of bad habits,like heavy smoking,eating too much sugar or not brush-ing teeth often.If your teeth are in bad shape, you should see a dentist and ask for help. They can help improve your teeth and tell you how to take care of then.
   Smiling can help you to feel more confident and make other people think of you as a more friendly person. If you want to keephealthy, you should not only smile but also laugh. An Indian doctor called Kataria said that young children should laugh about 300 times a day, wile adults should laugh between 7 and 15 times a day. So please take this chance to smile and laugh now!

    A crow(乌鸦) is about twenty inches (英寸) long and black all over. Crows are dirty birds because they live on bad food. Crows eat a lot of waste food. In this way they are more useful to us than any birds. They clean up the dirty things in our streets. Crows are always hungry. They look for food all day, and in the evening you can see them in large numbers flying back to their nests(巢) in the trees. They sleep there at night.

    Crows are much noisier than other birds. Very often a large number of crows will get together on one house and talk. Sometimes they talk together, and sometimes they do it by turns(轮流) like human beings(人类). It is easy to tell(判断) from crow's sound if they are pleased or angry. Perhaps crows talk a lot because they are friendly birds. A pair of crows will live together all their lives, and if one of them dies, the other one becomes very sad and quiet for the rest of its life and some time later it dies too.

    Do you know why different animals or pests have their special colors? Colors in them seem to be mainly used to protect themselves.

Some birds like eating locusts (蝗虫), but birds cannot easily catch them. Why? It is because locusts change their colors with the change of the colors of crops. When crops are green, locusts look green. But as the harvest time comes, locusts change into the same brown color as crops have. Some other pests whose colors are different form plants are easily found and eaten by others. So they have to hide themselves for lives and appear only at night.

    If you study the animals' life, you'll find the main use of colors is to protect themselves. Bears, lions and other animals move quietly through forests. They cannot be easily seen by hunters because their colors are much like the trees.

    Colors are useful not only on the land, but also in the sea. A kind of fish in the sea can give out a kind of black liquid (液体) when the fish face danger. The liquid spreads over quickly, so they cannot be found by their enemies and can quickly swim away. That is why they live safely though they are not strong at all.

    As space science develops, man has learned more and more about space. Space is not only amazing but also dangerous. While working in space, spacemen are facing danger.

    Scientists have found out that radiation(辐射) is the greatest danger to spacemen in space. When spacemen are working in space, they are in danger of radiation from the sun and other stars, which is bad for their health. Some special medicine may work a little, but there's no really helpful medicine.

    Space junk is also thought a great danger to spacemen. It's reported that there are 9,000 man-made things flying in space. About 30% of these are man-made satellites(卫星),10% are spaceships, and the rest are space junk. An explosion(爆炸) in 1996 made a cloud of 300,000 fragments(碎片), each at least 4 mm in size, and even a small piece of these knocked against a spaceship window and did some damage. Scientists are working on helpful ways to do with space junk.

    Although space is really dangerous, it interests everyone on the earth. In the near future, it may become usual for people to spend a few days in a space hotel. We may even choose to leave the earth and live in space for a few years. So we should care for space and do something to help improve the space environment.

    Dinosaurs, one of the most successful groups of animals that have ever lived, evolved(进化) into many sizes and shapes. The word "dinosaur" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles(爬行动物) ".

    Some things to keep in mind about dinosaurs：

    ·Not everything big and dead is a dinosaur. Dinosaurs were a special group of reptiles. Although Pterosaurs(翼龙) were similar, they were not true dinosaurs.

    ·Not all dinosaurs lived at the same time. Different dinosaurs lived at different times. Although you can see Stegosaurus(剑龙) fought against Tyrannosaurus

(暴龙) in some movies like King Kong and Jurassic Park, no Stegosaurus ever saw a Tyrannosaurus. It's because Tyrannosaurus appeared on the Earth about 80 million years after Stegosaurus died out.