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    There is good and bad news about the number of trees on out planet. The good news is that there are seven times more trees for everyone than we thought.

    A few years ago, scientists believed that the world bad 400 billion（十亿）trees. However, a new study from Yale University shows that there are around three trillion（万亿）trees. Thatmeans a three followed by 12 zeroes. That means there are about 420 trees for everyone in the world today.

    The bad news is that thousands of years ago, the earth had six trillion trees. Humans cut in half the number of trees on the planet. Take Europe for example, it was a large forest thousands of years ago. Now much of it is cities and towns. And another 15 billion trees are lost each year.

    Dr. Thomas Crowther is one of the scientists from Yale University. He said the new number of trees from their study would change nothing. " The study only shows we need to do more to protect our forests." He said.

    Taking away a city's rubbish is a big job. Every day tracks come into a city to collect it. Most rubbish is made up of things we can't eat or use. If we kept these things we would soon have a mountain of rubbish.

    In some cities the rubbish is collected and taken outside of city to a dump. Often the city dump is placed where the ground is low or there is a big hole. The kitchen rubbish is broken into small pieces and sent into the sewage system. The sewage system takes away the used water from toilets, bathtubs and other places.

    To keep mice and flies away, some earth is used to cover the newly dumped rubbish. Later, grass may be planted on the rubbish-filled land. Finally, a house or a school may be built there, and then you'd never know that this had once been an old rubbish dump.

    In other cities the rubbish is burnt in special places. The fire burns everything but the metal. Sometimes the metal can be used again in factories where things are made of metal. The food parts of rubbish are put in special piles where they slowly change into something called humus, which looks like black earth. It is rich with the kinds of things that feed plants and help make them grow.

    Do equations and historical( 历史的) facts give you a headache? A change in the school timetable(课程表) may be the cure.

    This  suggestion  is  based  on  a  recent  study  carried  out  by  scientists  from  Royal  Holloway, University of London, the UK. They found that teenagers could get higher marks in math if they studied in the mornings and better grades in history if they were taught in the afternoons.

    The researchers studied students' grade reports, class timetables and absence rates( 缺席率) at a middle school in Bulgaria over nine years. The results showed that when teenagers had math classes earlier in the day, they scored about 7 percent higher in tests than when taught in the afternoon. Lower-ability students feel stressed most when they were studying math in the afternoon. In history, students taught in the afternoon scored about 6 percent higher than when they were taught in the morning. However, there was no sign that similar patterns( 模 式 ) work for other subjects such as languages or geography.

    Velichka Dimitrova, who did the study, said the findings show that we perform( 表 现 ) better at different times of the day. “In the morning, our brains are better and fresher, so we are better at doing something like problem solving, where we require more speed and attention” Dimitrova told The Telegraph. “In the afternoon, it seems that this process slows down. On the other hand, history is more suitable for the afternoon, when we are more creative and open to discussion,” she said. Subjects such as foreign languages require both memory and creativity. This is perhaps why the time of day had no influence on language learning.

    Time-of-day influences such as these are believed to have something to do with the body clock.

    This is the system in our bodies that controls when we need to sleep. Some schools in the US and UK have tried later school starting times for teenagers, whose body clocks change as they grow into adults. Dimitrova believes schools should jump at this chance to improve their students' performance just by changing their timetables.

    Now, it seems that artficial intelligence （人工智能）is becoming more and more popular in our life, and it has greatly influenced the ways we live, work and play. Can you imagine that you can play table tennis with a robot one day?

    Recently, a scientist has invented a special robot that can be a table tennis coach. That means people can practice table tennis not only with humans but also with a robot.

    Named Forpheus, the smart table tennis robot is quite special. It can study the movements of human players and the speeds of the ball. In this way, it can exactly judge (判断) the player' skill levels (水平). Then it uses the information to change the ways to play. If the player are just beginners, Forpheus will play in a slow and easy way, but if the players are better ones, the robot will play in a faster and more difficult way. What's more, while playing, it encourages players to try their beat with words like "Good job!", "Come on!" and so on. At the same time, it gives some good advice to improve their skills. So it is both a good partner and a clever coach.

    Science and technology is developing quickly these days and it's hard to imagine what will happen in the future. "In the next 20 years, it will be possible that one robot teaches another to play table tennis or even invents another one!" Takuoya, the inventor of the robot, said.

    The world has a plastics problem. And not just with bottles. Straws (吸管), toothbrushes, toys—all kinds of plastics are placed in landfills. There are 5 billion tons of plastic waste in the world. Landfills have a large part of it. By 2050, they could have 13 billion tons.

    Much of plastic rubbish finds its way into the ocean. Scientists say 165 million tons of plastic pieces are floating around out there. And most plastic won't disappear itself. It will remain in the ocean forever. Whales get caught in plastic fishing nets. Birds die because of eating plastic straws and bottle caps. It endangers ocean life. Plastic rubbish even ends up in the food we eat.

    Governments are realizing that recycling may not be enough. They are taking more actions. Though it is hard work, it is still a solvable problem. In Britain, people are not allowed to use plastic bags except in hospitals. Scotland and several coastal cities in the US stop people from using plastic straws as well. The US alone throws away some 500 million plastic straws a day. That's enough to circle the Earth twice. "Saying no to a plastic straw is an easy way to make a big difference," Diana Lofflin who started www.strawfree.org said. "'Do we really want to pass on to our children a world harmed by plastic?"

    People have found some tasty ways to reduce waste. A company in India made the first-ever spoon that people can eat. You don't have to eat the spoon. You can throw it away. It decomposes (分解) in just a few days. David Edwards, a scientist at Harvard University, designed an eatable bag which can protect the food or drink inside. It is a delicious replacement for plastic ones.