Look up in the sky. Do you see any clouds? What do they look like ?Some clouds are fluffy（绒毛状的）and white, like cotton balls. Others are dark, almost black, and make you hold an umbrella. At sunrise and sunset, clouds can look red, purple, or yellow，as if they were colored with crayons. But what are clouds anyway?

   You can't reach out and touch the clouds, but you can make your own in a bottle.

   You need :

   An adult helper

   A huge, empty glass container（容器）

   A metal strainer（滤网）

   Hot water

   Ice

   Do these:

   Step1. Have your helper fill the container with hot water ,leave it there for two minutes, then pour out most of the water, leaving just an inch or two at the bottom of the container.

   Step2. It's your turn now. Put the strainer over the mouth of the container. Fill the strainer with ice. Watch the container.

What happened?

A cloud formed in the container! Some of the hot water at the bottom of the container turned into the hot water vapor. The water vapor rose and met with cold air coming off the ice. When the water vapor condensed（凝结）,it formed a cloud. Hot air rises ad carries with it lots of water vapor. The higher the air rises, the more it cools down. Soon the cold air can't hold all the water vapor so it starts turning into tiny water drops, and becomes a cloud.

    Can dolphins (海豚) talk? Maybe they can not talk with words, but they can talk with sounds. They show their feelings with sounds. Dolphins travel in a group. We call a group of fish a "school". They do not study, but they travel together. Dolphins are mammals (哺乳动物), not fish, but they swim together like fish in a school. Dolphins talk to the other dolphins in the school. They give information to the others. They talk when they are happy, sad or afraid. They say "welcome" when a dolphin comes back to the school. They talk when they play.

    They make some sounds above water. They make many more sounds under water. People cannot hear these sounds because they are very, very high. Scientists make tapes of the sounds and study them. Sometimes people catch dolphins for a large aquarium (水族宫). People can watch dolphins in a show. Dolphins do not like to be away from their school in an aquarium. They are sad and lonely. There are many stories about dolphins. They help people. Sometimes they save somebody's life. Dolphins' meat is good, but people do not like to kill them. They say that dolphins bring good luck. Many people believe this.

    We always think that it's OK for us to leave our plants home during our vacation as plants won't scream like pets, right?

    A recent study done by a group of scientists at the Tel Aviv University has discovered that some plants will scream when they are under stress.

    The research was carried out on tomato plants and tobacco plants by cutting their stems (茎) and depriving (剥夺) them of water. A microphone was placed 10cm away from them. When their stems were cut, the plants started "screaming" between 20 and 100 kilohertz, and the scientists guess that this scream is probably meant to warn other plants nearby.

    When the tomato plants' stems were cut, 25 ultrasonic (超声的) pain sounds were recorded in an hour, and at the same time, 15 ultrasonic pain sounds were recorded from the tobacco plants. When they were deprived of water, the tomato plants recorded even more pain sounds with 35 pain sounds in an hour while the tobacco plants recorded about 11 pain sounds.

    The plants also acted differently to different kinds of stresses. For example, the tobacco plants gave out a stronger "scream" when they were deprived of water than when their stems were cut. But plants that were in no immediate (立刻的) danger gave out less than one ultrasonic sound per hour.

    The group of scientists said that plants are not as quiet as we had always thought, but in fact their voices are so high-pitched that we just can't hear them.

    Last year, another study has found out that plants feel pain when they are touched or hurt and they will give off a terrible smell.

Why Live Near the River?

Imagine pioneers traveling west in the 1800s. In those days, it was hard to find running water and shops where to buy things. As time went by, most pioneers had to make their homes near a river so that they could get water easily.

How did rivers help people live in the past?

Back in the pioneer days, people grew their own food, caught their own fish, and collected their own water.{#blank#}1{#/blank#}People also used rivers as roads, traveling on boats. Traders shipped goods (货物) to people from other areas.

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When building many factories right next to rivers for the water's power to make machines work, some people choose to live near a river for working. They use the river to ship large goods, and some of them work at these factories where power factories provide electricity for homes and businesses.{#blank#}3{#/blank#}

Who wouldn't want to live near a river?

Some people are nervous about living near rivers because rivers can cause floods (洪水). Every once in a while, floodwaters rise higher than a house, ruining (毁坏) many things in the house.{#blank#}4{#/blank#} Such waste causes water to smell bad and pollutes the crops (作物) we eat. Water and crops in polluted areas can make people very sick.

How do people decide where to live?

 {#blank#}5{#/blank#}Rivers can provide jobs and fun activities in a beautiful place. However, those living by rivers should be willing to accept possible problems as well.