Rik-rrork. That's the sound made by frogs singing at night. You might have thought the noises were just the musical sounds of ure. You probably didn't realize that you were listening to frogs' communicating with one another. These familiar sounds are not random(随意的) but are actually meaningful to frogs.

Being Heard but Not Seen

    Frogs are good at being heard but not seen. These animals have several characters that protect them n predators (捕食者 ). One is their size. Most frogs are small, which helps them hide easily. Many frogs have coloring that allows them to hide in their environment, making it difficult for predators to find them. In addition, most frogs are active at night. So it is difficult for most animals to see them in the dark.

Small but Loud

    Frogs call to tell other frogs where they are. Their voices can be very loud. But when frogs feel the footsteps of a larger animal, they become silent.

    Many of the world's frogs live in rain forests, where there are many different animals calling to their own species (物种 ) in their own ways. Frogs need to call loudly enough to be heard through the noise in order to find other frogs.

Singing with Purpose

      Peter Narins is a scientist who studies how frogs call to one another. He has learned that frogs use their calls to help them tell friend from enemy. They sing the same notes at the same volume (音量 ) each time they call. In addition, Narins has found that frogs of the same species always sing the same patterns (模式 ). These patterns are like a code that is different from other animals' calls and difficult for others to copy. Narins also observed that timing plays a role in frog calls. Many frogs call only at a certain time of night.

More Work to Do

    Scientists want to better understand the purpose of frog calls. Studies continue to explore the details of frogs' unique communication. Being heard but not seen is one way frogs stay safe. The idea that they have found a way to survive in a dangerous world is simply amazing.

    In ancient Chinese stories, the moon is home to Chang'e and her pet rabbit Yutu. Now, Chang'e and Yutu are really there!

    On Dec. 2, 2013, the Chang'e-3 lunar probe（月球探测器）took off from Sichuan, China. On Dec. 14, 2013, it landed successfully on the moon. Several hours later, Yutu, China's first moon rover（月球车）, "walked" out of the probe and onto the moon.

    Yutu is a big robot that weighs 140 kg. It is about 1.5 meters long and 1.1 meters tall. It has six wheels and travels at 200 meters per hour.

    There are cameras on both Yutu and Chang'e-3. They can take pictures of each other. That's how Yutu got is first picture on the moon.

    Yutu and Chang'e-3 will explore（探索）the moon.

    Yutu can walk around and work for three months. It can look for natural resources（资源）on the moon. Chang'e-3 can work at the landing site for one year and it can watch the space from the moon.

    They both get energy from the sun. So they "get up" to work when the sun rises, and "go to sleep" at night.

    China is now the third country in the world to soft-land on the moon after the US and the former Soviet Union.

    Also, China is planning to send its Chang'e-5 lunar probe around 2017. The probe will fly back to the Earth and bring natural resources from the moon.

    When will China fly a human to the moon? There's no plan yet. But the door is now open.

    Tablets are really useful devices, but their big screens always make them as a burden to carry around without a bag. Wouldn't it be great if there were a phone with the powers of a tablet that could be folded up and fit into the hand?

Now something like a tablet-shaped but foldable phone is about to become available. In February, South Korean electronics company Samsung and China's Huawei both showed foldable phones, The Galaxy Fold and Huawei Mate X respectively. Mobile phone use has entered the "foldable future", The Verge noted.

    The technology could change our lives in unusual ways. These devices, due to their foldable screens, give us the larger screens we want. At the same time, they still fit easily into the pocket. As USA Today noted, they're "the combination of a small tablet and smartphone, all in a single device".

    The technology could change other devices too. For example, we could make TVs that stick to walls like posters, or fold up easily to hide away in drawers. In crowded modern cities, they will help us to make full use of available space.

    In a keynote speech, Samsung's senior vice president of mobile product marketing, Justin Denison, called the foldable screen "the foundation for the smartphone of tomorrow". "It's a piece of white paper for us to paint something beautiful on together," he said.

    So is there nothing to stand in the way of the foldable future?

According to technology news website Android Authority, the necessary displays* were difficult to produce. At present, these foldable devices cost too much. For example, the price of Huawei Mate X is 17,500 RMB. That's a price that few people will be able to afford.

    But if the foldable device isn't going to change the world overnight, there is no doubt that it is coming.

    Patrick Moorhead, an industry expert told The Verge, "Few are debating if foldable mobile displays are the future of smartphones; the only question is when and by whom."

    In 1845, a deadly disease struck the farms of Ireland, killing all the Lumper potato plants. In another place or time, the death of single crop species (物种) might not have been so important. But in Ireland, in 1845, people depended almost solely on the potato for food. The death of one species caused a terrible famine(饥荒). Now, some scientists are worried that such a famine could happen again, but on a much wider scale.

    Over the centuries, farmers have discovered thousands of different species of food crops. Each species has special qualities. Some can be grown in very hot or cold climates. Others are not affected by certain diseases. However, you won't find many of these species in your local supermarket. To feed the seven billion people on Earth, most farmers today are growing only species of plants and farming only species of animals that are easy to produce in large numbers.

    For example, in the Philippines, there were once thousands of varieties of rice: now fewer than 100 are grown there. In China, 90 percent of the wheat varieties grown just a century ago have disappeared. Scientists believe that over the past century, we have allowed more than half of the world's food varieties to disappear.

    One solution to this problem is to collect and store the seeds (种子) as many different plant varieties as we can before they disappear. The idea was first suggested by Russian scientist Nikolay Vavilov. In the 1920s and 1930s, he collected around 400,000 seeds from five contients. More recently, others are continuing the work he began.

    In the U.S. state of Iowa, Diane Ott Whealy wanted to protect historic plant varieties, like the seeds her great –grandfather brought to the U.S. from Germany more than a hundred years ago. She and her husband started a place called Heritage Farm, where people can store and trade seeds.

    More importantly, the people at Heritage Farm don't just store the seeds; they plant them. By doing this, they are reintroducing foods into the marketplace that haven't been grown for years. These food species are not just special in terms of appearance or taste. They also offer farmers food solutions for the future, from the past.

    All week long, it seems you sit inside at school when it is fine outside. But when the weekend comes, it is very cloudy. There is rain according to(根据) the weather report.

    Scientists studied more than 40 years of weather data(数据) from around the world. It shows weekend weather is different from weekday weather in some places.

    Part of the study involved(包括) 660 weather stations in the US. At more than 230 of these places, the weather on Saturday, Sunday and Monday was different from the weather on Wednesday, Thursday and Friday. The difference was small, but the pattern was striking enough to make the scientists take notice.

    "This kind of weekly rise and fall doesn't line up with any natural cycles, "the scientists say. Instead, they think that air pollution from human activities makes the weather become different.