Researchers have discovered the first Earth-sized planet. It was first marked by NASA's Kepler telescope(望远镜) so it's called Kepler-186f. It lies about 500 light years from Earth and goes around its star. The planet is the right distance from its star for water: not too close or not too far. Water is one important condition that scientists guess is necessary for life. So it could have water and possible life. It's called a true Earth cousin.

"This planet is an Earth cousin, not an Earth twin," said Barclay, who is among a team of scientists reporting on the discovery in the magazine Science this week.

"It's very exciting to find a planet similar to the Earth," Barclay said, "It's not easy work because things change as we get more measurements."

Scientists don't know anything about the air of Kepler-186f, but it will be a task for future telescopes which can study for chemicals that have something to do with life.

    "It's possible for life to live in this planet, but that doesn't mean there is life in it," Barclay said.

So far, scientists have found nearly 1,800 planets in the universe.

"The past year has seen a lot of progress in the search for Earth-like planets. Kepler-186f is very important because it is the first planet that is the same temperature and is almost the same size as Earth," scientist David Charbonneau wrote in an email.

    Why do plants grow in some places but not in others? Why does some land have so many plants growing on it, while other land has almost no plants growing on it at all?

    To grow, plants need several things. One is warmth. In very cold places almost nothing grows. Plants also need water. In very dry parts of the Earth only a few unusual plants can grow. That's why there are almost no trees or grass in dry deserts.

    Plants must also have a place to put down their roots and grow. They find it difficult to grow on hard land. The town is built(被建立) on hard land. The plants there have only the soil between the cracks of the stones to grow in.

    Another thing plants must have before they can grow is food. What will happen if we try to make things grow on the sandy beach? A few plants, such as grass, will grow in sand, but most plants won't. Even if the weather is warm enough and we water the plants each day, many of them will die because the sand on this beach has almost no food for plants.

    A study of sleep in sparrows suggests the relationship between sleep and the ability to learn may be more complicated (复杂) than realized．

    Niels C. Rattenborg of the University of Wisconsin—Madison wanted to find out how the sparrows were able to deal with getting so much less sleep．He and his team brought eight wild birds into a lab and watched them for one year．They invented a game to check how well the birds could learn．In the game, the sparrows had to peck (啄) three buttons (按钮) in a certain order to get a food treat．The scientists discovered that the birds' ability to learn the right button order depended on (依靠) two things, the time of year and the

amount of sleep the birds had．

    During the migration (迁徙) season, the sparrows were restless at night and got much less sleep than usual．Even so, they were able to understand how to get the food quickly as if they'd had a regular night of sleep．Outside the migration season, they found the sparrows that got less sleep than usual had much more difficulty learning how to get the food treats than birds that had a regular sleep．

    The results show that the sparrows can deal with much less sleep during the migration season than usual．If scientists can find out why, they may learn from sparrows and find ways of helping people deal with getting much less sleep．Until scientists fully understand the relationship between sleep and learning, it's better to get plenty of sleep when preparing for an exam．

    Sheridan is a town in Indiana, USA. There are lots of trees near the public schools in the town. Their leaves turn sunlight into energy（能量）.

    In Sheridan, the sun not just provides energy for plants. It keeps schools running, too. The schools use solar panels（太阳能电池板） to turn sunlight into energy.

    The science behind solar energy is not new. But Sheridan Community Schools is the first school district(学区) in Indiana to be completely solar-powered. The district finished putting in solar panels for all three of its schools in 2016. Some of the panels can turn to follow the sun across the sky.

    In 2008, fewer than 1. 000 schools used solar power. By 2014, there were 3, 727 schools with solar panels in the USA. These numbers come from a report by the Solar Foundation. Roxie Brown, a program director there said that the number of schools with solar panels has continued to go up since 2014.

    Solar energy has some advantages over other energy sources(来源). Sunlight is a renewable source, which means it won't run out. Solar panels don't harm the environment. Also, sunlight is free. So by using solar power, schools can save money on energy costs.

    Brown thinks more schools should run on solar energy. "The sun is Earth s energy source," she says. "We're trying to make it the energy source for our houses cars and schools, too."

    But using solar energy isn't always easy. Solar panels can be expensive to put in. The panels also take up space. But for many schools, going solar is worth it.

    Can plants eat people? Probably not, but there are many plants that eat meat. Some of them are big. And they can eat small animals. One famous meat-eating plant is the Venus flytrap(捕蝇草).

    The Venus flytrap is a very strange plant. It grows in dry parts of the United States. Its leaves are like the pages of a book. They can open and close very quickly. Inside the leaves, there are three small hairs. If a fly touches one of the hairs, the leaf closes quickly. The fly cannot get out. In about half an hour, the leaf presses the fly until it is dead. Then, the plant covers the fly. Slowly, the plant eats the fly.

    Why do plants do it? Most plants get what they need from the sun, the air, and the ground. In some places, the ground is very poor. It doesn't have all these important things, especially nitrogen(氮). Animal meat has a lot of nitrogen, so some plants eat meat to get what they need. Let's hope that some of the bigger plants don't get the same idea!

    In most parts of the world, many students help their schools make less pollution. They join "environment clubs". In an environment club, people work together to make our environment clean.

    ●No-garbage lunches. How much do you throw away after lunch? Environment clubs ask students to bring their lunches in bags that can be used again. Every week they will choose the classes that make the least garbage and report them to the whole school!

    ●No-car day. On the no-car day, nobody comes to school by car—not the students or not the teachers! Cars give pollution to the air, so remember:

    Walk, jump, bike or run.

    Use your legs! It's lots of fun!

    ●Turn off the water! Did you know that some toilets can waste twenty to forty liters (升) of water an hour? In a year that would fill a small river! In environment clubs, students mend those broken toilets.

    We love our environment. Let's work together to make it clean.