Did you ever notice your shadow at different times of the day? Shadows change as the day goes on.

    In the morning the sun makes long shadows. In the middle of the day, the sun seems to be right over your head and your shadow is very, very small. As the sun goes down in the afternoon, shadows grow very long again. But the long afternoon shadows fall in a different direction from the long morning shadows.

    Long, long ago people told time by the shadows. Put a piece of paper in the sunlight. Stand a pencil in a spool in the middle of the paper. The shadow of the pencil will fall on the paper and move as the day goes on. Use a watch and every hour marks the place where the shadow falls. Now your paper is a sundial（通过太阳辨别时间的日规）and you can tell every sunny day. Just look at your paper sundial and see where the shadow falls.

    The year is 2094. It has been announced that a comet (彗星) is heading towards the Earth. Most of it will miss our planet, but two pieces will probably hit the southern half of the Earth.

    On 17 July, a piece four kilometers wide enters the Earth's atmosphere with a massive explosion. About half of the piece is destroyed, but the remaining part hits the South Atlantic at 200 times the speed of sound. The sea boils and a huge hole is made in the sea bed.  Huge waves are created and spread outwards from the hole. The wall of water, a kilometer high, rushes towards southern Africa at 800 kilometers an hour. Cities on the African coast are totally destroyed and millions of people are drowned.

    Before the waves reach South America, the second piece of the comet lands in Argentina. Earthquakes and volcanoes are set off in the Andes Mountains. The shock waves move north into California and all around the Pacific Ocean. The cities of Los Angeles, San Francisco and Tokyo are completely destroyed by earthquakes. Millions of people in the southern half of the earth are already dead, but the north won't escape for long. Because of the explosions, the sun is hidden by clouds of dust, temperatures around the world fall to almost zero. Crops are ruined. The sun won't be seen again for many years. Wars break out as countries fight for food. A year later, no more than 10 million people remain alive.

    Could it really happen? In fact, it has already happened more than once in the history of the Earth. The dinosaurs were on the Earth for over 160 million years. Then 65 million years ago they suddenly disappeared. Many scientists believe that the Earth was hit by a piece of object in space. The dinosaurs couldn't live through the cold climate that followed and they died out. Will we meet the same end?

    Plastic waste has polluted the Arctic. Two new studies have spied bags, fishing rope and tinier bits of rubbish in the Barents Sea. This sea sits north of Norway, Sweden, Finland and Russia. It mixes with the Arctic Ocean, which is even farther north.

    Plastic waste in the Arctic could harm wildlife and may hint that large volumes of human rubbish are collecting there, says Melanie Bergmann. She is one of the scientists who spotted the waste. She studies Earth's oceans at the Alfred Wegener Institute in Bremerhaven, Germany. She first started counting bits of plastics in the Barents Sea because she kept spotting signs of the stuff there in images taken with deep-sea cameras.

    Bergmann and her colleagues counted pieces of plastic from an icebreaker, a boat designed to break through large blocks of ice in very cold waters. They also tracked plastic pieces they saw during helicopter rides over Arctic waters. The team found 31 pieces of plastic. “That doesn't seem like much, but it shows us that we've really got a problem, one that extends even to this remote area, far from civilization,” Bergmann says. She and her colleagues described their findings October 21 in Polar Biology.

    Another team has also been counting plastics in the area. Those scientists took water from the Barents Sea and counted the number of smaller bits of plastics, called microplastics.

    Plastic in the ocean is dangerous to animals. Some may get caught in rope or bags. And wildlife may swallow bags and other plastic bits. That makes them feel full. But some may eventually starve because they are not getting the nutrients they need to live. Sometimes plastics also may break down in an animal's body and release poisonous chemicals. If another animal later eats the one that swallowed plastic, it too can end up with poisonous chemicals in its body. This, in turn, can travel up the food web, endangering predators (肉食动物) — even people.

Seeds on Ice

    Close to the North Pole, remote and rocky Plateau Mountain in the Norwegian archipelago of Svalbard seems an unlikely spot for any global effort to safeguard agriculture. In this cold and deserted environment, no grains, no gardens, no trees can grow. Yet at the end of a 130-meter-long tunnel cut out of solid stone is a room filled with humanity's most precious treasure, the largest and most diverse seed collection—more than a half-billion seeds.

    A quiet rescue mission is under way. With growing evidence that unchecked climate change-will seriously affect food production and threaten the diversity （多样性） of crops around the world, the Svalbard Global Seed Vault （地窖） represents a major step towards ensuring the preservation （贮藏） of hundreds of thousands of crop varieties. This is a seed collection, but more importantly, it is a collection of the traits found within the seeds: the genes that give one variety resistance to a particular pest and another variety tolerance for hot, dry weather.

    Few people will ever see or come into contact with the contents of this vault. In sealed boxes, behind multiple locked doors, monitored by electronic security systems, enveloped in below—zero temperatures, and surrounded by tons of rock, hundreds of millions of seeds are protected in their mountain fortress. Frozen in such conditions inside the mountain, seeds of most major crops will remain viable for hundreds of years, or longer. Seeds of some are capable of retaining （保留） their ability to grow for thousands of years.

    Everyone can look back now and say that the Seed Vault was a good and obvious idea, and that of course the Norwegian government should have approved and funded it. But back in 2004, when the Seed Vault was proposed, it was viewed as a crazy, impractical, and expensive idea.

    We knew that nothing would provide a definite guarantee. But we were tired, fed up, and frankly scared of the steady, greater losses of crop diversity. The Seed Vault was built by optimists who wanted to do something to preserve options so that humanity and its crops might be better prepared for change. If it simply resupplied seed gene banks with samples those gene banks had lost, this would repay our efforts.

    The Seed Vault is about hope and commitment-about what can be done if countries come together and work cooperatively to accomplish something significant, long-lasting, and worthy of who we are and wish to be.

    It is widely accepted that nightmares (噩梦) are a reaction to negative experiences that happen during waking hours. However, some scientists believe that nightmares do have some real benefits. One 2017 study, for example, found that frequent nightmare sufferers rated themselves as more empathetic (有同理心的). They also displayed more of a tendency to unconsciously mirror other people through things like yawning People who have constant nightmares also tend to think further outside the box on psychoanalysis tasks. Some other researchers have found support for the idea that nightmares might be linked to creativity.

    People seeking cure for nightmares were not necessarily more fearful or anxious, but rather had a general sensitivity to all emotional experience. Sensitivity is the driving force behind intense dreams. Heightened sensitivity to threats or fear during the day results in nightmares, whereas heightened passion or excitement may result in positive dreams. And both these forms of dreams may feed back into waking life, perhaps increasing suffering after nightmares, or promoting social bonds and empathy after positive dreams.

    The effects go further still. This sensitivity overflows into awareness and thoughts －people who have a lot of nightmares experience a dreamlike quality to their waking thoughts. And this kind of thinking seems to give them a creative edge. For instance, studies show that such people tend to have greater creative talent and artistic express and people who often have nightmares also tend to have more positive dreams than the average person.

    The evidence points towards the idea that, rather than disturbing normal activity, people who are unfortunate in having a lot of nightmares also have a dreaming life that is at least as creative, positive and vivid as it can be distressing and terrifying. What's more, this imaginative richness is unlikely to be limited to sleep, but also is filled with waking thoughts and daydreams. Even after people wake up and shake off the nightmare, in other words, a mark of it stays behind, possessing them throughout the day.

    Moving smoothly and silently through Alibaba Group Holding Ltd's futuristic "FlyZoo" hotel, black disc-shaped robots about a metre in height deliver food and drop off fresh towels.

    The robots are part of a set of high-tech tools that Alibaba says strongly cuts the hotel's cost of human labour and eliminates the need for guests to interact with other people.

    Formally opened to the public last month, the 290-room FlyZoo is an incubator for technology Alibaba wants to sell to the hotel industry in the future and an opportunity to showcase its prowess in artificial intelligence. It is also an experiment that tests consumer comfort levels with unmanned commerce in China.

    Inside the hotel, softly-lit white panelled walls bring to mind the interiors of Hollywood spaceships. Guests check in at podiums that scan their faces, as well as passports or other ID. Visitors with a Chinese national ID can scan their faces using their smartphones to check in ahead of time.

    Elevators scan guests' faces again to verify which floor they can access and hotel room doors are opened with another face scan.

    "It's very quick and safe. I haven't used it much yet, but basically, I can be in my room in one minute, "said guest Tracy Li. Li added that safety was one of her priorities and she was pleased her room could only be entered with a scan of her face.

    In the rooms, Alibaba's voice command technology is used to change the temperature, close the curtains, adjust the lighting and order room service.

    At the hotel's restaurant, taller capsule-shaped robots deliver food that guests have ordered via the FlyZoo app while at a separate bar, a large robotic arm can mix more than 20 different types of cocktails. Facial recognition cameras add charges to the room rate automatically.

    The hotel does employ humans, though Alibaba declined to detail how many. This includes chefs and cleaners as well as reception staff, who will assist with conventional check-in procedures for guests unwilling to have their faces scanned and want to use electronic key cards.