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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.

    How much do you know about the bird world?

    Birds use three gaits: walking, running and hopping．

    In walking, the left and the right feet move in turn (轮流)．Each foot is on the ground for more than half the time, so there are more times when both feet are on the ground．In running, each foot is on the ground for less than half the time, and there are more times when both feet are off the ground．In hopping, the feet move almost at the same time．This means that for much of the time both feet are off the ground．

    Middle-sized and large birds walk at low speed and either run or hop at higher speed．Small birds, such as sparrows and swallows, usually do not walk, because walking may be too slow for them．It seems that the more time a bird spends in trees, the more likely it is to hop．Perhaps there is wide space in a tree between one thin branch (树枝) and the next, so walking is not a good choice, while on each small branch there is enough room to set both feet down side by side, so hopping seems to make sense．

    Space vegetables are grown from seeds(种子) that have been taken to space and brought back to the earth. The seeds are affected by the radiation(辐射)and low gravity in space. When they are brought back to the earth, these seeds produce vegetables that are bigger and healthier than normal vegetables. However, some people worry about eating space vegetables. They think that space vegetables might not be good for us and could make us get sick because of the radiation in space. However, people should not be frightened because space vegetables are very healthy.

    Here are some facts that you should know about space vegetables.

Space vegetables are grown from seeds that are carefully chosen. When seeds are brought back from space they are tested to make sure that they will be safe to eat.

    Space vegetables are better for you than normal vegetables. For example, space tomatoes stay fresh for twenty days, which is one week longer than normal tomatoes。

    After genetically modified (转基因)food appeared in the market, people worried that they were eating unknown things. For example, if nut genes(坚果基因) are put inside potatoes, people allergic(过敏的) to nuts might get sick from eating these potatoes because they do not know they are also eating nuts. Unlike genetically modified food, space vegetables have not been genetically changed. This means that no new genes are put into the vegetables. Therefore, there are no dangers of eating unknown.

    When your pen is broken, the battery in your toy runs out, or you have some leftover food, what do you do with these things? You will probably throw them all into one trashbin. But actually, all of these pieces of trash need to be sorted (分类) separately.

    Garbage sorting is a big problem worldwide. In recent years, some Chinese cities have been working hard on it.

    Shanghai has worked with Alipay to create a "green account (账户)" service for its residents (居民). Account holders get points by correctly sorting their garbage. Through the Alipay app, they can exchange the points for milk, phone cards and other products. The city is asking all of its residents to sort their garbage into four groups: wet, recyclable (可回收的), harmful and dry.

    Wet waste is also known as household waste. "They are things you don't want but that pigs can eat," Guangzhou Daily explained.

    Paper, metal (金属), glass and other things that can be reused are recyclable waste.

    Harmful waste includes things like medicine, batteries and fluorescent bulbs (荧光灯泡).

    Finally, any waste that's not wet, recyclable or harmful will go in the "dry waste" bin.

    Many other Chinese cities are also using this method to sort their garbage. For example, Shenzhen has been sorting its garbage into the same four groups since 2012. Students there also receive waste-sorting guidebooks that they must study.

    China is improving its waste-sorting efforts. There is still a long way to go. But it's never too late to learn how to sort your trash properly and protect the environment.

    If you don't sort your garbage, all of it will go to a landfill (垃圾填埋场) and be buried together. These landfills can take up large areas of ground that could have been used for planting trees. The electronic waste you throw away, such as batteries, can pollute the soil and water. Other pieces of garbage, like the metal part of a pen, can be used to make other things if they are properly recycled.