The word “day” has two meanings. When we talk about the number of days in a year, we are using “day” to mean 24 hours. But when we talk about day and night, we are using “day” to mean the time between sunrise and sunset. Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转）of the earth. At the equator（赤道）day and night are sometimes the same length（长度）. They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock in the evening. For six months the North Pole（北极）is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere（半球）. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

    When it comes to long-distance space travel, the biggest problem is that spaceships can only get as far as the fuel onboard allows. In 1964, a British writer Clarke came up with the idea of "solar sailing "in his science fiction. He wrote of large, mirror-like sails that collect power from the sun and free spaceships from the limitation of fuel.

    This sci-fi idea was first successfully brought to reality (现实)in 2010 by Japan's IKAROS satellite. Ever since then, the technology has improved mission by mission, with the latest being NASA's Advanced Composite Solar Sail 3 (ACS3).NASA has recently announced that it's going to team up with Lithuania's satellite-building company NanoAvionics to test its new sail in low-Earth orbit.

    To many, a solar sail (太阳帆) may look like a giant solar panel (太阳能电池板), but it actual works very differently. While solar panels take in energy from the sun and change it into power solar sails use photons(光子) given off by the sun to create direct power. Besides, solar panels are big and heavy to carry, while solar sails are just the opposite.

    According to Abbott, the CEO of NanoAvionics, the upcoming low-Earth orbit trial with NASA, if successful, will be followed with deep-space missions that involve larger solar sails. These are ideal for low-cost and long-distance space travel because fuel will no longer be a problem.

    "It started as a sci-fi dream, "Popular Mechanics reporter Jennifer Leman wrote. "Now it could be the future of space flight."

    The longest river in the world is the Nile. It is 4,160 miles long. But the Nile is not the mightiest (浩荡的) river. The Amazon is. The Amazon is also the second longest river in the world. It is 3,900 miles long. How long is that? It is longer than a road going from New York City to San Francisco.

    The Amazon is the mightiest for this reason. It has the greatest amount of water. More water runs out to sea than from the Nile. Mississippi, and Yangtze rivers combined. That's a lot of fresh water! The force of its current (水流) is great too. The current can be seen 200 miles out in the sea. There's a story to show the Amazon's amazing current. A ship sailed (航行) far away from Brazil. It ran out of drinking water. Another ship passed by. The first ship asked for water. Someone on the second ship said. "Just put your buckets (桶) into the sea."

    Here are a few more facts. At its mouth, the Amazon is 90 miles wide. Even up the river, you can't see the opposite bank. The river is also very deep(from 22 to 76 meters). During rainy season, the river rises about 16 meters more. Big ocean ships can sail about 1,800 miles upstream (向上游). Smaller ships can sail 1,000 miles farther.

    What's the most bloodthirsty fish in the world? No. It's not the shark It's the Amazon's piranha.It is only about 10 cm to 20 cm long. It has sharp teeth. Thousands travel in a group. No animal can swim across the river. A 1,000-pound horse would be just bones (骨头) in three minutes. Are you planning a trip to the Amazon? Here's a piece of advice. Leave your swimsuit at home!

    We have known for a long time that flowers of different plants open and close at different time of day. This is so familiar that there seems to be no need to ask the reason for it. Yet no one really understands why flowers open and close like this at particular times. The process (过程) is not as simple as we might think, as recent experiments have shown. In one study, flowers were kept in constant (连续的) darkness. We might expect that the flowers, without any information about the time of day, did not open as they normally do. In fact, they continued to open at their usual time. This suggests that they have some mysterious (神秘的) way of knowing the time. Their sense of time does not depend on information from the outside world; it is, so to speak, inside them, a kind of "inner clock".

    This discovery (发现) may not seem to be very important. However, it was later found that not just plants but also animals —including man —have this "inner clock" which controls the working of their bodies and influences (影响) their activities. Men, then, are also influenced by this mysterious power. Whether we wish it or not, it affects such things in our life as our need for sleep, our need for food.

    In the past, this did not matter very much because people lived in natural condition. In the modern world, things are different; now there are spacemen, airplane pilots and, in ordinary life, a lot of people who have to work at night. It would be very useful, then, to know more about the "inner clock". Such ordinary things as flowers might help us understand more about ourselves.

    Have you ever had trouble sleeping in a new place?

    Lots of people do. And now researchers from Brown University in Rhode Island think they know why. They found that one-half of the brain "remains more awake" than the other half when people are trying to sleep in a new place.

    This appears a case of the brain keeping people ready for trouble in a new place, the researchers said.

    The sleep findings were reported in Current Biology by Brown University. In their report, the researchers said many people report they have a harder time sleeping the first night at a hotel or other places outside their home. They call it "first-night influence". "In Japan they say, ‘If you change your pillow, you can't sleep,'" said Yuka Sasaki, one of the report's writers, "You don't sleep very well in a new place. We all know about it."

    The researchers measured brain waves (测量脑波) for 35 volunteers over two nights in a laboratory. The two nights were a week apart. They found during the first night the left half of the brain was more active than the right half. This was during the first deep-sleep period, the researchers said.

    Sasaki said a lot of questions remain.

    Researchers did not keep measuring brain waves all night long. So, they don't know if the left half keeps "watch" all night. Or whether it "works in shifts" with the right half later in the night. They also don't know why the brain activity, at least during the first period of deep sleep, is always on the left half.

    For some, this research may be calming. It is good to know that our brain is "looking out for us" in a new place. But it may not help with sleep. That brain activity, at least according to this new research, makes it harder to get the sleep people need to wake up well rested in the morning.

    In many parts of the world, spring is in full bloom. Flowers of all colors and shapes can be seen everywhere. They make our world look and smell better. So, it is no surprise that people love flowers! Well… except for one —dandelions (蒲公英)!

    There is probably no flower more hated or misunderstood than the bright yellow dandelions. In order to have a tidy garden, we try our best to kill all dandelions or dig them out—only to see these unwanted plants grow right back. But that's not bad because we have good reasons to leave them alone.

    In fact, the most hated flower on Earth is one of the healthiest to eat. A 2014 study lists dandelions as one of the top 41 healthiest foods. Coming in at number 16, the wild growing "weed" topped popular healthy like broccoli and kale. Dandelions are full of vitamins K, A, E and C. They have long been used as medicine.

    Another great thing about dandelions is that you can eat every part of them—from the bottom of the roots to the top of the flower, either cooked or uncooked. You can also put the flowers or roots in hot water to make dandelion tea. Great! Nothing goes to waste!

    Besides, dandelions are fun for kids! Pick the white top, close your eyes, make a wish and blow! If you blow all of it in one breath, your wish will come true. This game is played all over the world.

    Still unwanted plants? Of course not! Dandelions are good for human health as well as our kitchen. They also offer kids free toys! So next time when you are blowing them, blow harder—you're helping this great plant grow in the wild, even if your own wish does not come true!

    The Taojinshan Greenway (绿道) in Shenzhen has been open to citizens since July, 2019. Sitting at the west side of Shenzhen Reservoir, it is as long as 7.07 kilometers.

    With plants along both sides and lakes lying at different sites, tourists can enjoy wonderful scenery all the way. If they are tired, they are free to take a rest at the 3 stops that were built for breaks. Additionally, five unused guard posts (岗享) along the old route have remained. They serve as sightseeing platforms for visitors to feel the cultural spirit behind the place.

    Of all its 13 scenic spots, 8 were designed to have Al helpers. From them, visitors can gain information on parking, tour guide services and more. The smart greenway, with free Wi-Fi available, also provides environmental date, such as temperature, wind speed and air quality. What's more, it can give visitors wamings of coming extreme (极端的) weather and natural disasters, including rainstorms and typhoons that may hit Shenzhen.

    "Started in the middle of 2015, the building process of the Taojinshan Greenway has taken good care of de wildlife and environment nearby" said Yang Shiming, an offcial in charge of the project."

    The greenway is an example of construction projects without doing harm to the nature. It also shows the role that technology plays in building a smart greenway for more life convenience."

    Facial recognition technology (人脸识别技术) is widely used today. For example, police use it to search for bad guys. It's also used to unlock phones or doors.

Now, a similar technology, known as facial detection (探测), is entering a new field: the business of advertising (广告).

    Such systems are going through tests in a small number of stores. One facial detection system can judge a person's "happiness" or "fear" level. That helps businesses see how people feel as they lo at products or advertisements in the store. Another system can detect whether someone is wearing eyeglasses. If so, stores can send advertisements for new glasses to him.

    Facial detection technology is also placed inside some large advertising boards. Cameras in those boards can detect information like people's ages and five levels of feelings——from "very happy" to "very unhappy".

    Supporters of the technology say it could improve buyers' experience by showing products they might like or by offering them products at lower prices.

    Some are against the idea of using such systems. Pam Dixon, the head of a privacy (隐私) group, is one of them. She thinks      . For example, a business could raise the price of a product according to a person's age and feelings.

    However, one store testing the technology explained they wouldn't store people's information like names or ages. And the advertising board system maker said it didn't keep any personal information or record any videos.

    In China, each year we produce about 150 million tons of city waste according to a report. Where does the waste go? People usually burn or bury (埋) it. On the other hand, we receive waste from some developed countries. We get useful materials from it. What about the useless part? Still burned or buried. But is it a proper way?

    Waste pollutes the environment, harms people's health and causes animals to die. It also gets in the way of green and sustainable development. We can't make money at the cost of the environment.

    We should leave clean water and green mountains to our next generations (一代人).

    China is getting really serious about waste now.

    We are taking less waste from developed countries. From January 2018, China started to stop 24 kinds of waste coming into our country. And more laws and rules about dealing with waste have been made. Anyone who breaks the law will be punished.

    China also tries to put waste into good use. One example is the waste electricity factories. They burn waste to make electricity in a green way. No harmful gases or water will get into the environment.

    The Chinese government is encouraging people to do waste sorting(分类). Cities like Shanghai,

    Chongqing and Beijing have their own sorting systems. More will join them soon. As President Xi Jinping said during a visit to Shanghai, waste sorting is a new fashion (时尚). More and more people are forming the habit of separating their waste.

    Many people dream of traveling into outer space. However, spending too much time in space may cause health problems, according to NASA. That's why astronauts shouldn't spend more than six months on the International Space Station(ISS)at a time.

    Muscle(肌肉)and bone loss

    In a weightless environment, astronauts don't need to use their muscles to support themselves, so their muscles start shrinking. To keep their muscles strong, astronauts need to exercise about 2 hours each day on the ISS.

    Sleep disorder (混乱)

    In space, the sun rises every 90 minutes. It is hard for astronauts to get used to this light-dark cycle when they sleep. Also, they need to sleep in a sleeping bag that is fixed to a wall, and it is uncomfortable.

    Astronauts now have individual sleeping pods(分离仓)that help shut out some of the light. Sill, studies have found that most astronauts only sleep about six hours each night, even though they should have eight and a half hours.

    Brain changes

    Scientists have found that the position of the brain changes during spaceflight. It can affect how the brain deal with how we see, stay balanced and move around. That's why some astronauts can't control their movement and balance after returning to the Earth. Scientists have also found that astronauts may get used to this change after going on more spaceflights. But they still need to do more research.