Modern graffiti began in big cities in the United States in the 1970s. In New York, young people wrote their names, or 'tags', in pen on walls around the city.

    One of the first 'taggers' was a teenager called Demetrius. His tag was TAKI 183. He wrote his tag on walls and in stations in New York. He did it just for fun and he had never imagined his behavior would have launched an amazing art campaign. Other teenagers saw Demetrius's tag and started writing their tags too. Soon, there were tags on walls, buses and trains all over New York.

    Then, some teenagers started writing their tags with aerosol paint. Their tags were bigger and more colourful. Aerosol paint graffiti became very popular in the 1970s and 1980s. It appeared on trains, buses and walls around the world.

    In the 1990s and 2000s, a lot of graffiti artists started painting pictures. Some artists' pictures were about politics. Other artists wanted to make cities beautiful and painted big, colorful pictures on city walls.

    In some countries, writing or painting on walls is a crime. Sometimes, graffiti artists have problems with the police. In other countries, artists can draw and paint in certain places. For example, in Taiwan, there are 'graffiti zones' where artists can paint on walls. In São Paulo in Brazil, street artists can paint pictures on walls and houses. Their pictures are colorful and beautiful. Some tourists visit São Paulo just to see the street art!

    In Bristol in the UK, there is a street art festival in August every year. Artists paint all the buildings in a street. Lots of people come to watch the artists and take photos. You can see exhibitions of street art in some galleries too. There have been exhibitions of street art in galleries in Paris, London and Los Angeles.

    Are you sometimes a little tired and sleepy in the early afternoon? Many people feel this way after lunch. They may think that eating lunch is the cause of the sleepiness. Or, in summer, they may think it is the heat. However, the real reason lies inside their bodies. At that time—about eight hours after you wake up—your body temperature goes down. This is what makes you slowdown and feel sleepy. Scientists have tested sleep habits in experiments where there was no night or day. The people in these experiments almost always followed a similar sleeping pattern. They slept for one long period and then for one short period about eight hours later.

    In many parts of the world, people take naps in the middle of the day. This is especially true in warmer climates, where the heat makes work difficult in the early afternoon. Researchers are now saying that naps are good for everyone in any climate. A daily nap gives one a more rested body and mind and therefore is good for health in general. In countries where naps are traditional, people often suffer less from problems such as heart disease. Many working people, unfortunately, have no time to take naps. Though doctors may advise taking naps, employers do not allow it! If you do have the chance, however, here are a few tips about making the most of your nap. Remember that the best time to take a nap is about eight hours after you get up. A short sleep too late in the day may only make you feel more tired and sleepy afterward. This can also happen if you sleep for too long. If you do not have enough time, try a short nap—even ten minutes of sleep can be helpful.

    You probably know who Marie Curie was, but you may not have heard of Rachel Carson. Of the outstanding ladies listed below, who do you think was the most important woman of the past 100 years?

    Jane Addams (1860-1935)

    Anyone who has ever been helped by a social worker has Jane Addams to thank. Addams helped the poor and worked for peace. She encouraged a sense of community(社区) by creating shelters and promoting education and services for people in need. In 1931, Addams became the first American women to win the Nobel Peace Prize.

    Rachel Carson (1907-1964)

    If it weren't for Rachel Carson, the environmental movement might not exist today. Her popular 1962 book Silent Spring raised awareness of the dangers of pollution and the harmful effects of chemicals on humans and on the world's lakes and oceans.

    Sandra Day O'Connor (1930-present)

    When Sandra Day O'Connor finished third in her class at Stanford Law School in 1952, she could not find work at a law firm because she was a woman. She became an Arizona State Senator(参议员) and in 1981, the first woman to join the US Supreme Court（高等法院）. O'Connor gave the deciding vote in many important cases during her 24 years on the top court.

    Rosa Parks (1913-2005)

    On December 1, 1955, in Montgomery, Alabama, Rosa Parks would not give up her seat to a white passenger. Her simple act landed Parks in prison. But it also set off the Montgomery bus boycott. (抵制行动) It lasted for more than a year, and kicked off the civil-rights movements. "the only tired I was, was tired of giving in." said Parks.

    The United States of America is faced with serious traffic problems. Too many cars travel on the freeways to and from the city. This heavy traffic causes delays and lots of pollution from cars that are using their engines but not moving anywhere. Many cars going nowhere is called a "traffic jam".

    While many cities in the United States have traffic problems, some of the worst are in Los Angeles, California, which is a big city with a large downtown area. One of the reasons why there is so much traffic is that few people use public transportation to get around.

    Experts say one possible solution to the traffic problems in Los Angeles would be to make people want to use the public transportation system of buses instead of their own cars. People say the buses should be made easier, faster, less costly and more dependable than driving a car.

    One suggestion is to reduce the cost of taking the bus. Also, Los Angeles could make the buses easier for people to use. The buses could travel to more places in the city. And there could be more special lanes on the freeways that only buses could drive in. These changes would make taking the bus faster and easier than driving a car.

    Other experts have said that the best way to ease traffic problems is to charge people money to drive on the freeways. These charges, or tolls, would be for the most crowded roads. The tolls would be in effect during the times of a day when most people drive to work and home. These times in the morning and late afternoon are called "rush hour". People would try to avoid driving on these roads, which would decrease the amount of traffic. Also, the money collected from these tolls could be used to improve the roads and public transportation system.

    By the end of the century, if not sooner, the world's oceans will be bluer and greener thanks to a warming climate, according to a new study.

    At the heart of the phenomenon lie tiny marine microorganisms (海洋微生物) called phytoplankton. Because of the way light reflects off the organisms, these phytoplankton create colourful patterns at the ocean surface. Ocean colour varies from green to blue, depending on the type and concentration of phytoplankton. Climate change will fuel the growth of phytoplankton in some areas, while reducing it in other spots, leading to changes in the ocean's appearance.

    Phytoplankton live at the ocean surface, where they pull carbon dioxide (二氧化碳) into the ocean while giving off oxygen. When these organisms die, they bury carbon in the deep ocean, an important process that helps to regulate the global climate. But phytoplankton are vulnerable to the ocean's warming trend. Warming changes key characteristics of the ocean and can affect phytoplankton growth, since they need not only sunlight and carbon dioxide to grow, but also nutrients.

    Stephanie Dutkiewicz, a scientist in MIT's Center for Global Change Science, built a climate model that projects changes to the oceans throughout the century. In a world that warms up by 3℃, it found that multiple changes to the colour of the oceans would occur. The model projects that currently blue areas with little phytoplankton could become even bluer. But in some waters, such as those of the Arctic, a warming will make conditions riper for phytoplankton, and these areas will turn greener. "Not only are the quantities of phytoplankton in the ocean changing." she said, "but the type of phytoplankton is changing."

    Sharon, aged 22

    The most important thing to keep in mind when you're going into high school is to be yourself. Besides, some high school teachers will not care about things like how much homework you have already had in one night. It's best to just learn to deal with things and manage your time wisely so you can achieve everything you need to.

    Frank, aged 21

    I think almost every kid feels both nervous and excited before their first day of high school. You will probably love it. I know I did. You should join some sports or activities that will make your high school experience more enjoyable. Good luck!

    Eddie, aged 20

    When I started high school, I was really nervous too, especially since I had been homeschooled all through middle school and didn't really know anyone. I suppose the best advice would be to just relax. The first couple of days can be a little bit hard, but things will become easier before you know it.

    David, aged 19

    I'm not going to lie. The first day is kind of frightening. But you'll get used to it. Don't be afraid of anyone; some students may pick on (故意刁难) you more if you let them know you're afraid. Just take it easy. Making some friends and staying with them will greatly help you get used to high school quickly. After the first week, it's really not bad at all. Don't worry.