Living in the wild can be hard. Finding food and staying safe aren't easy. Each day, animals struggle to survive in their habitats. Not all animals get by on their own. Some animals form a close partnership with other kinds of animals. These pairings are called symbiotic (共生的) relationships.

    In a symbiotic relationship, the animals depend on each other. One animal helps the other meet its needs. Sounds good, right? Not always. Some animals are not very kind to their partners. In some cases, one animal meets its needs but hurts its partner. Take ticks, for example. These insects suck blood to live. To get blood, they attach themselves to other kinds of animals. Ticks can pass germs that cause disease instead of helping their hosts.

    In other relationships, animals don't treat their partners so poorly Both animals benefit from living with the other animal.

    Small animals called cleaner shrimps have found a way of helping fish at coral reefs. As their name suggests, the shrimps clean the fish. They hang out at what scientists call a cleaning station. A fish stops by. Then a shrimp climbs onto the fish and even steps into the fish's mouth. The shrimp uses its tiny claws to pick stuff off the fish's body. That can include dead skin, tiny pieces of food, and wee creatures that can hurt the fish. The fish gets a nice cleaning. The shrimp enjoys a tasty meal offish trash.

    Small birds called plovers are also in the cleaning business. They have big customers—crocodiles. Crocs have long snouts（鼻子）filled with sharp teeth, Cleaning them is tricky. When a croc opens its mouth, the plover hops right in. The croc does not snap its snout shut. Instead, it lets the plover eat small, harmful animals attached to its teeth. The plover gets an easy meal while the croc gets clean teeth.

根据短文内容，请将单词填写在题号对应的横线上。

    Traveling to Europe to see its famous monuments like the Eiffel Tower in Paris and the Colosseum(古罗马竞技场) in Rome，is not exactly the perfect vacation for kids. Fortunately, Europe has more to offer than lots of old churches and ancient history. You can enjoy some fun with your kids in the sun at some of the famous beaches of the continent.

    Dubrovnik, Croatia .

    There's nothing more attractive than the beautiful beaches bordering Dubrovnik, Croatia's most beautiful city. It's surrounded by stone walls like a castle. It almost makes you feel like you're suntanning(晒黑皮肤)at Hogwarts, where the hero Harry Potter of the story is trained!

    Sicily, Italy

    Rabbit Beach on the island of Sicily offers shallow and crystal clear waters, making it a perfect beach for families with young children. And if you want to add interest to your experience, try snorkeling(徒手潜水).

    Nice, France

    Lined with palm trees and first-class hotels, the city of Nice, located on the French Riviera, offers a whole coastline of Mediterranean beaches. Because they're in the heart of the city, these beaches attract a large crowd. So if you're looking for something more private, you'll have to travel a few miles outside of the city center.

    Ribadeo, Spain

    At low tide, Playa de las Catedrales in Ribadeo, Spain is the perfect beach to take in the natural wonders without urban amusements like restaurants, bars or huge crowds. It boasts beautiful wild flowers and unique rock formations ma king it truly a picture worthy.

    A black hole is created when a large star burns out. Like our sun, stars are unbelievably hot furnaces(熔炉) that burn their own matter as fuel. When most of the fuel is used up, the star begins to die.

    The death of a star is not a quiet event. First there is a huge explosion. As its outer layer is blasted off into space, the dying star shines as brightly as a billion suns.

    After the explosion, gravity pulls in what's left of the star. As the outside of the star sinks toward the center, the star gets smaller and smaller. The material the star is made of becomes tightly packed together. A star is so solid that a teaspoon of matter from it weighs billions of pounds.

    The more the star shrinks(收缩), the stronger the gravity inside it becomes. Soon the star is very tiny, and the gravity pulling it in is unbelievably strong. In fact, the gravity is so strong that it even pulls light into the star! Since all the light is pulled in, none can go out. The star becomes black when there is no light. Then a black hole is born!

    That's what we know about black holes. What we don't know is this: What happens inside a black hole after the star has been squeezed into a tiny ball? Does it keep getting smaller and smaller forever? Such a possibility is hard to imagine.

    But if the black hole doesn't keep shrinking, what happens to it? Some scientists think black holes are like doorways to another world. They say that as the star disappears from our universe, it goes into another universe. In other words a black hole in our universe could turn into a "white hole" in a different universe. As the black hole swallows(吞噬) light, the white hole shines brightly--somewhere else. But where? A different place, perhaps, or a different time--many years in the past or future.

    Could you travel through a black hole? Right now, no. Nothing we know of could go into a black hole without being crushed(挤压). So far the time being, black hole must remain a mystery.

Black holes are a mystery--but that hasn't stopped scientists from dreaming about them. One scientist suggested that in the future we might make use of the power of black holes. They would supply all of Earth's energy needs, with plenty to spare. Another scientist wondered if a black hole could some day be used to swallow earthly waste--a sort of huge waste disposal(处理) in the sky!

    A bicycle may be seen by most people as just another vehicle (交通工具), but for many Londoners, cycling is a way of life. According to an official UK government survey in 2017, about 570,000 bike journeys are made every day in London. In fact, almost half of the vehicles that pass over London Bridge each day are bikes.

    London introduced a public bike-sharing system in 2010. "There can be no doubt that our trusty bicycles have changed the way people get around our great city," Johnson told the Guardian in 2015.

    In London, bikes are used for more than just taking short trips to and from the subway. No matter where you want to go in the city, taking a bike is usually the quickest and easiest choice.

    And it's not just shared bikes that the government is encouraging people to ride. In many companies across the UK, the UK government's Cycle to Work scheme (骑行倡议) allows employees to buy a brand new bike without having to pay any tax (税). This means that it's common to see many people cycling to and from work, and some employers even provide workplace showers and lockers (储物柜) for their workers. More importantly, a cycle-friendly boss may let you off for being late if you rode a bike to work.

    Not only is it great for the environment and our body, cycling is also good for the mind. According to National Geographic Magazine, "Bike riding can improve people's happiness."

    Agatha Christie used to go out at night. She will never forget the night when she met a robber(盗贼) many years ago.

    That evening, she was invited to a birthday party which lasted till 2 o'clock in the morning. Without company, Agatha walked home along the quiet street. Suddenly from the shadow of a dark building, a tall man with a sharp knife in his right hand ran out at her. "Good morning, lady," the man said in a low voice, "I don't think you wish to die here!" "What do you want?" Agatha asked.

    "Your earrings (耳环). Take them off!" Agatha suddenly had a bright idea. She tried to cover her necklace with the collar (衣领) of her overcoat while she used the other hand to take off both of her earrings, and then she quickly threw them on the ground.

    "Take them and let me go." she said. The robber was uncertain. He saw that the girl didn't care for the earrings at all, only trying to protect the necklace. He realized the necklace would cost more, so he said, "Give me your necklace."

    "Oh, sir. It's not worth much. Please let me keep it."

    "Stop talking rubbish. Quick!"

    With shaky hands, Agatha took off her necklace. As soon as the robber disappeared, she picked up her earrings and ran as fast as she could to her home.

    The earrings cost 480 pounds and the necklace the robber had taken away cost only six pounds ten shillings (先令).

    The saying, "Don't judge a book by its cover, means you should not guess the worth or value of something based on how it looks. That message was clear at a recent event called the Human Library Project. The event took place at the Northern Virginia, or NoVa, Community College, outside of Washington. D. C.

    The Human Library began 19 years ago in Denmark. It grew from a youth organization called "Stop the Violence". Today, it is a worldwide movement. At the NoVa event, students got the chance to learn from a person—a "human book"—instead of a library book.

    Patricia Cooper organized the event. She said that human books celebrate variety by telling their life stories in an easy-going setting. "The goal of the human library is to talk to people in your community who you may otherwise not speak to because you have your own prejudices (偏见) and hopefully to break down some of these barriers."

    This is the third year that NoVa has held such an event. The collection of human books included a civil rights activist, a scientist from the American space agency NASA, and an opera singer.

    Artist Brian Dailey was a human book. He spoke about his travels to 113 countries in seven years. Dailey said that, during his travels, he asked people whom he took pictures of for a one-word answer to a series of other words—such as love, freedom and war. He discovered that people in different countries often had very different reactions to the same word.

    When Dailey asked people in Africa about the word "war" they used words like justice, liberation and peace. When he asked the same question to people in Syrian refugee camps, the answer was: "tears, hunger, fear, destruction".