Tornadoes(龙卷风) are one of the worst weather events on earth. Each year, tornadoes kill many people. The storms often carry homes, cars and trees from one place to another. And they can also destroy anything when they pass by.

      A tornado comes from a thunderstorm(雷雨) in the sky down to the ground. It is big at the top and small at the bottom.

      A tornado appears when winds in different directions meet in the cloud and begin to turn in circles. People can see tornadoes almost everywhere in the world. But most happen from late winter to middle summer. There is a second high season in November. During spring, warm air moves north and mixes with the cold air left from winter. In November, the opposite happens. Cold air moves south and mixes with the warm air left from summer.

      Tornadoes always come suddenly. Weather scientists have done something to predict(预测) tornadoes. But the storms often move too fast for people to protect themselves. Last year, tornadoes killed more than one hundred people in United States.

      Scientists say the best place to be in is a small room, without windows, in the middle the lowest part of a building when tornadoes come.

    According to a national study, in the summer of 2013, the long-term(长期的) warm weather in the UK has made the number of butterflies increase.

    Around 46,000 volunteers(志愿者) took part in the year's Big Butterfly Count(统计), and they found 830,000 butterflies.Volunteers were asked to make a record.They would record how many times they found one of 21 different kinds of butterflies.

    Later, the scientists used the records to find out how many butterflies of different kinds were found across the country.Then, they compared the results to the year before.They found that there was a rise in the number of 15 kinds of butterflies, and some of them went up by 50% compared to recent years.They said the increase in butterfly numbers was a result of warm weather.

    However, scientists also said that the number of butterflies was in decline(下降) as a whole, and that we might not see the same result the next year.

    Richard Fox, who works for Butterfly Conservation, said, “UK butterflies are in long-term decline.Studies have been done since the 1970s.They show that UK butterflies have declined in type and number.”

    He added, “We must halt the long-term decline of these beautiful creatures(生物).The only way is by stopping the damage that has been done to the butterflies' habitats(栖息地) across the UK.”

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    On a very hot day，it can be hard to know what to do to feel good．If we run around，we may feel dizzy and sick to our stomachs．This is because our body can't get rid of the extra heat．However，if we just sit and do nothing，we may feel restless and bored．Are there things we can do to feel good when we are hot? Yes! Playing in water is one way to feel cooler．Of course，people with a pool，river，lake or ocean nearby can play in the water very conveniently．People don't have these can still play in water too．Taking a cool bath or shower can feel good on a hot day．On hot days，you can lose two gallons(加仑)of water in one day，so many doctors agree that you should try to drink at least 16 glasses of water to keep the water balance in your body． Mixing a little juice into your water is a good way to help you drink plenty of water．The food we eat also makes a difference in how we feel in the heat．Cold foods，such as ice cream and drinks with some ice can cool down our bodies．Fruits and vegetables are easy for the body and refreshing．Plan a healthy menu for a hot day．

    For thousands of years, humans have explored(探索)the Earth. Nowadays, we are exploring space. Astronomers (天文学家) are the modern﹣day explorers, Now, many astronomers are looking for new planets and new places for humans to live in the future. But where do astronomers start looking？

    First of all, astronomers look for a star. That's because our own Earth moves around a star (the Sun). More importantly, it is the correct distance( 距离) from the Sun for heat and light. So when astronomers have found the star, they look at the planets around it. In recent years, astronomers have found nearly 400 new planets with stars. However, many of these planets are either too near to the star or too far away.

    However, if the planet is in a good position, astronomers look for three key things: water, air and rock. Water is important because all life needs water. Humans can drink it and they can also grow plants with water. And plants produce air for humans to breathe and food to eat. Rock on a planet is also important. That's because there is often water under the rock.

    After many years of scrutinizing, astronomers have found a planet that is similar to the Earth. It's Gliese 581g and it's near a star. The astronomers think it has water and rock and the average(平均的) temperature is between -31℃ and -12℃.That's cold, but not colder than Antarctica or the Arctic Circle, for example. Gliese 581g is bigger than the Earth. A year on Gliese 581g is only 37 Earth days instead of 365. But astronomers do not think these are big differences and some of them think Gliese 581g will be a new Earth. However, Gliese 58Ig is twenty light years from the Earth.

    Many scientists think that the Earth is getting warmer and warmer. If the change continues, the natural world and human society will face danger. Scientists believe human activity is responsible for the climate change. We produce too many greenhouse gases. They keep heat in the air and make the Earth warm.

    Many of the world's governments agree. And they want to solve the problem. So from 7 to 18 December, 2009，they held a conference in Copenhagen, Denmark to talk about global warming and climate change.

    Representatives(代表) from 192 countries and regions went to the meeting. Although they have not reached an agreement, the Copenhagen Accord(《哥本哈根协议》) was an important beginning to save the Earth. It was made by a small group of countries including China，the USA，India, Brazil and South Africa. Some other countries at the conference also showed support.

    What are the key points of the Copenhagen Accord? The accord limits global temperature increases to 2C by 2050. Scientists think it could help the world to avoid serious climate change.

    The accord also says that developed countries must promise to cut greenhouse gas emissions(排放) to the UN by 31 January, 2010. The accord asks developed countries to give money to help developing countries.

    Many countries and organizations are disappointed that the conference didn't reach a legal document(法律文书). And they think the accord is not clear enough about its goals.

African leaders are worried that the goal of a 2℃ global temperature increase isn't good enough, because it will mean a much higher increase in temperature across Africa. This could cause big problems like food and water shortages across the continent.

    Still, UN Secretary General believes the conference was an "improvement".

    "Finally we reached a deal," Ban said," the accord may not be everything everyone had hoped for, but this is an important beginning."