What is color？ Why do some objects look red， others green， others blue？
Color is caused by reflected light rays. We see color because objects reflect light. Something that is red reflects mostly red light. （It reflects a little green of blue light， too， but we do not see it.） In the same way， a green object reflects mostly green light. White objects reflect all colors of light Black objects do not reflect any light.
What happens to the colors of light that are not reflected？ They are absorbed by the object. The darker the color， the less light is reflected and the more light is absorbed. Light that is absorbed is turned into heat. For this reason， dark-colored clothes are warmer in the sunlight than light-colored clothes.
The sum spent on books and magazines seems small. Most families take at least one daily newspaper and a Sunday paper. They may take several weekly magazines， usually including the Radio Times （《无线电节目报》）。 This gives them the weekly programmers of the B.B.C. The small sum spent on books does not mean that people do very little reading. Millions of cheap， paper-backed（平装的） books are bought every month. There are good public libraries everywhere from which books may be borrowed. Over one million books are taken out from these libraries every week-day. In some homes， however， there is less serious reading now than there was fifty years ago. Many people prefer to listen to the radio， or look at films or television. Many people seem to prefer popular picture magazines to magazines that contain serious reading.
Astronomers（天文学家） can tell just how hot the surface of the moon gets. The side of the moon toward the sun gets two hundred degrees hotter than boiling water. The night side reaches 243 degrees below zero.
In the eclipse（月食）， the earth‘s shadow fails on the moon. Then the moon’s temperature may drop 300 degrees in a very short time.
A temperature change like this cannot happen on the earth. Why does it happen on the moon？ Astronomers think that the surface of the moon is dust. On the earth， rocks store heat from the sun. When the sun goes down， the rocks stay warm. But the dust of the moon cannot store heat. So when the moon gets dark， the heat escapes quickly. The moon gets very cold.
Water is very important to living things. Without water there can be no life on Earth. All animals and plants need water. Man also needs water. We need water to drink， to cook our food and to clean ourselves. Water is needed in offices， factories and schools. Where else is water needed？
Water covers about seventy percent of the Earth‘s surface. There is water is seas， rivers， lakes， springs（泉） and wells.
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 oceans and lakes and streams is a liquid. The water in the air is not a liquid but a gas. We call it water vapor.
Clouds are made of water. They may be made of tiny drops of water. They may also be made of snow crystals. Snow crystals are tiny crystals of ice. Ice is frozen water or water that has become solid.
2 用于购买书籍杂志的钱看来很少，大多数家庭至少订一份日报和一份周日版的报纸。他们也可能订几份周刊，通常包括《无线电节目报》。这就可以使他们知道 BBC每周的节目。在书籍上开销少，并不是说人们不怎么看书。每月都出数以百万计的平装廉价书。各处都有良好的公共图书馆，可以从中借书。这些图书馆每个工作日都借出一百多万册书籍。然而比起五十年前，有些家庭读书不那么认真了。许多人情愿听广播或者看电影或电视。许多人似乎情愿看通俗画报，而不看刊载严肃读物的杂志。
1 Finding enough meat was a problem for primitive man （原始人） . Keeping it for times when it was scarce （短缺的）was just as hard. Three ways were found to keep meat from spoiling： salting， drying and freezing.
People near salty waters salted their meat. At first they probably rubbed dry salt on it， but this preserved only the outside. Later they may have pickled （腌制） their meat by soaking it in salt water.
In hot， dry lands， men found that they could eat meat that had dried while it was still on the bones. They later learned to cut meat into strips and hang it up to dry in the hot air.
Men in cold climates found that frozen meat did not spoil. They could leave their meat outside and eat it when they pleased.
2 Strange things happen to time when you travel， because the earth is divided into twenty-four time zones （时区）， one hour apart. You can have days with more or fewer than twenty-four hours， and weeks with more or fewer than seven days.
If you make a five-day drip across the Atlantic Ocean（大西洋）， your ship enters a different time zone every day. As you enter each zone， the time changes one hour. Traveling west， you set your clock back； traveling east， you set it ahead. Each day of your trip has either twenty-five or twenty-three hours.
If you travel by ship across the Pacific（太平洋）， you cross the international date line（国际日期变更线）。 By agreement， this is the point where a new day begins. When you cross the line， you change your calendar one full day， backward or forward. Traveling east， today becomes yesterday； traveling west， it is tomorrow！
3 Volcanoes have been erupting（喷发） on the earth for millions of years. More than five hundreds still erupt today. These are called active volcanoes. Volcanoes are located in belts or chains. They are found where the earth‘s crust is weak. The weak spots let the hot rock escape when the volcano erupts.
Many volcano belts are mountain ranges along the edges of continents. One pest runs along the western coast of South American up through tire western part of the United States. Other volcanoes are found in ocean basins（盆地）。
About three-fifths of all active volcanoes in the world are in the Pacific Ocean（太平洋）。 Many of these volcanoes erupt under the water. The Hawaiian Island（夏威夷群岛） were built by volcanoes that began erupting under water and finally reached the surface of the ocean.
4 In order to provide a home， food and clothing for himself and his family， every worker needs a job at a reasonable wage. He joins a trade union（工会） because the trade union helps him. An important duty of the trade union is to help their members when they are unable to work because of illness， or accidents at work， and when they become too old to continue working. In the early days of the trade union movement， before the State provided health and unemployment insurance and old age pensions（养老金）， this kind of help was much more important than it is today.
Every member of a trade union pays a small sum of money each week， called his subscription（会费）。 This covers the coast of organizing the union， and provides the found to help the members. From this fund members also receive help if they go on strike against their employers， or are locked out（解雇） by them.
For every man in the Civil War（内战） who died in battle， two or three man died of disease， Doctor of that time knew very little about causes of sickness or ways of preventing it. Thousands of men in poor health became soldiers. Hundreds of others had never had childhood diseases. Many of these soldiers could not withstand the epidemics（传染病） of measles（麻疹）， mumps（流行性腮腺炎）， and whooping cough（百日咳） that went through the camps.
Army life was hard. Soldiers got few fruits or vegetables. There was no milk unless they happened to find a cow. Neither their clothes nor their shelters protected the troops from rain， snow， and cold. Sickness and disease were spread by insects， rats， and impure drinking water. Often the men drank straight from muddy streams.
Gunshot wounds（枪伤） were serious， ns in any war， but they did not cause as much death and suffering as disease did.
In ancient times no women allowed to take part in any sports. In the Victorian era（维多利亚时代）， people thought it was unlady-like for a woman to be involved in any sport activity.
There have been changes in the view of women in the modern world. These changes have allowed many more women to participate in sports. Many women now take part in sports and games of different kinds and enjoy doing so.
Women also are more active in sports now because of two changes. First， in the early twentieth century， women began to compete regularly in the Olympics. Also， the number of events for women in the Olympics has been growing quite steadily. In more recent years， many women‘s competitions have been on TV. Seeing women in professional sports on TV has made certain sports much more popular among women， as amateurs（业余运动员） and professionals（专业运动员）。
Can machines be as intelligent as human beings？ There is a lot of discussion these days about artificial（人工的） intelligence machines and their relation to human intelligence. People in many different occupations（职业） are involved in this discussion. They want to know how artificial intelligence will improve their lives. They also want to know how artificial intelligence will improve their lives. They also want to know if “intelligent” machines can be harmful to them. They would like to know what scientists predict for their future.
Human beings are a unique species. They have more complex brains than any other beings. They have the gift of speech and can communicate their thoughts， ideas， and feelings. Human beings are interested in the past and can learn from past experiences. They study their present problems carefully and try to find the best solutions. They look to the future and plan their next activities. They invent tools and machines to help them in their work.
Do you want to say what you think in a letter to the President of the United States？ You‘ll get a reply from him-written in ink， not typed-after only a few days.
The President gets about 4，000 letters every week. He answers everyone who writes to him on special White House（白宫） paper. But he doesn‘t need a lot of time for it. In fact， he only gives 20 minutes a week to looking at his personal correspondence. He has the most modern secretary in the world to help him.
It‘s a computer， which has its own rooms on the first floor of the White House. It has a bank of（一堆，许多） electronic pens which write like the President writes， in his favorite light blue ink， Each letter the President receives gets a number， according to the type of answer it needs. The pens then write the correct reply for it， according to the number. Each letter takes less than a second to write.