When the weather is hot, you go to a lake or an ocean. When you are near a lake or an Ocean, you feel cool. Why? The sun makes the earth hot, but it cannot make the water very hot. Although the air over the earth becomes hot, the air over the water is still cool. The hot air over the earth goes up. Then the cool air over the water moves in and takes the place of the hot air.

    Then you may ask, "Why is the ocean full of salt?" Scientists will say that the salt comes from the rocks. When the rocks get very hot or very cold, they crack (爆裂). Rain falls into the cracks. The rain then carries the salt into the earth and into the rivers. The rivers carry the salt into the ocean. And then you may doubt, "But the ocean does not get more salty every year. Scientists are not sure about the answer to this question.

    We know a lot about our world. But there are still many answers that we do not have, and we are curious about them.

Today, many students are always stressed out. They are often too busy to think{#blank#}1{#/blank#} their diets and exercise. Healthy eating, along with regular(定期的) exercise {#blank#}2{#/blank#} (keep) you healthy. Do you think it is the only way to keep us in {#blank#}3{#/blank#}(healthy)?

As we know, it is important to give your body the energy it needs. Teenage girls {#blank#}4{#/blank#} (need) about 2,200 calories(卡路里) a day and boys need a bit more. {#blank#}5{#/blank#} (fiftieth) percent of your calories should come from rice, bread, {#blank#}6{#/blank#}(vegetable) and fruit. You also need to d{#blank#}7{#/blank#} a lot of water, six to eight glasses a day. Enough water will improve your skin and give you healthy hair.

Exercise can help to make you look good, feel good {#blank#}8{#/blank#} be healthy. Scientists suggest that teenagers should spend at{#blank#}9{#/blank#}(little) 30 minutes exercising every day.

Exercise can make you feel {#blank#}10{#/blank#} (relax)and energetic. It can even help you sleep better at night.

Let's eat healthily and take more exercise.

    A new rubbish sorting system (垃圾分类系统) has been put into use in Fangchengyuan Community in Beijing. Local people not only sort their rubbish, but also put QR codes (二维码) on it.

    "When the rubbish is collected and the codes are seen, I will get reward points to exchange for small gifts and even some money," one of the local people told reporters. "It's really creative and encouraging."

    This smart system has been carried out in 350 communities in Beijing. The capital of China has joined a list of cities in the country that take action to support rubbish sorting. For example, Shenzhen and Shanghai have made laws to punish people and organizations for not sorting rubbish correctly.

    China produces quite a lot of rubbish every year. Much of the rubbish is buried (填埋) in soil or burned without being sorted. Landfills (填埋场) take up a lot of land and have a risk of polluting the soil and water nearby. And burning rubbish can produce harmful gases. Through rubbish sorting, we can reduce the use of landfills and air pollution.

    What's more, rubbish sorting saves resources, which could bring economic benefits (经济利益). For example, a ton of waste paper can be reused to make about 850 kg of paper, saving 17 trees and 50 percent of water. Because of the benefits, some countries have developed successful sorting systems.

    Japan has a strict and detailed system. When people throw away a water bottle, the cap, the wrapper (包装纸) and the bottle itself have to go into three different bins. Every year, families receive special rubbish sorting timetables and directions from their local governments. They are advised to follow the timetables and directions when they throw away rubbish.

    In Australia, every family is provided with three rubbish bins—the red lid (盖子) bin for "general waste" like food and plastic bags, the yellow lid bin for "recycling" like steel and glass, and the green lid bin for "green waste" such as grass and leaves. On the streets, the bins are printed with pictures of the things that are allowed inside. It makes recycling quite easy.

    Many students think that learning a new language is very difficult. But if your brain (大脑) is only the same size as a bird's, you will know how difficult it is to learn English. That is what some birds can do.

Many birds can copy (模仿) the sounds of language. A type of African parrot is very good at doing this. This kind of parrot has grey feathers and people say they are clever.

Every December in London, the National Cage and Aviary Bird Show tries to find the best "talking" bird in the world. One bird named Prudle stood out (突出) among the " talking birds"  by winning this prize every year from 1965 to 1977. In 1958, somebody took Prudle from his nest in Uganda, and then sold it to a woman named Iris Frost. She then took care of him at her home in Seaford, England. Before he died in 1994, aged 35, Prudle knew almost 800 words in English. Prudle was also the oldest bird in the world that lived in a cage.

Another parrot named Puck was tested in 1993. It turned out that Puck knew even more words than Prudle. Puck knew more than 1,700 English words. In the 2003 Guinness Book of World Records, Puck was listed as knowing more words than any other bird in the world.

Many people take running as a way to keep fit. They like to run in hot weather and enjoy the feeling of being wet through with sweat (汗) while running. Cold weather during winter months, with low temperatures and freezing cold winds, may keep people from running in the open air.

However, a new study shows that the drop in temperature is a good reason to run. In fact, running in cold weather helps improve one's performance. Many runners might find it easier to run in winter than in hot weather. The body needs less water on a cold day than in warm weather. In addition, lower temperatures reduce stress on the body. When you run in cold weather, your heart rate is lower.

John Brewer is a professor of applied sports science at St. Mary's University in London. For this study, he and other researchers put a group of people into a room where summer and winter weather conditions are recreated. The test subjects were asked to run 10,000 meters under both conditions and their biological measurements were recorded and studied.From the study, every movement runners make produces heat. The heat can be really damaging unless the body can lose it. One way in which people lose heat is by sweating. The body loses heat through sweat. The body also loses it by transporting the blood to the surface of the skin and it puts more stress on the heart, especially in hot conditions because it's much harder to lose heat when the outside environment is warm. As the result shows, running in hot temperatures results in 6 percent higher heat rates.

When running in cold conditions, the body does not have to work hard to move blood to the skin surface. Therefore, it requires less energy. The most important thing for people who run in cold conditions is to wear the right clothing. Keep as dry as possible when exercising in low temperatures. Wear warm and light clothing to keep sweat away from your skin. So don't let winter weather keep you indoors. Simply get ready for the low temperatures and start running.

What did people do to communicate in ancient times? Here are some very interesting answers to this question.

Alphorns（阿尔卑斯长号角） were once used to communicate in the mountains in Switzerland. They are very long and they are very heavy, too. So, alphorns are hard work to blow. Then why use them? Well, they make a loud sound which travels much further than the human voice. Herdsmen（牧民） used them to call cows in the distance for milking. At sunset, alphorns were also used to send a message down to villagers that all was well. These days, they're mostly used as musical instruments.

The people of La Gomera, an island in Europe, have their own special way of sending messages from mountain to mountain. They use a whistled（口哨） language called el silbo, which comes from their language Spanish. Whistling can be heard within five kilorneters, which means it can be used across much greater distances than shouting. El silbo can be used for all kinds of messages, including greetings and announcements. El silbo is used a lot less than it used to be, but primary school students in La Gomera have to leave it.

Did you know it 's also possible to send messages without making a noise? This is called visual communication, and it includes the use of flags and even smoke. Long ago, soldiers along the Great Wall of China used smoke as a warning. If they noticed the enemy from their signal（信号） tower, they lit a smoky fire. When soldiers in other towers saw the smoke signal, they lit fires, too. In this way, messages about the enemy's arrival could travel as far as 800 km in just a few hours.