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    Rivers are one of our most important natural resources(资源). Many of the world's great cities are located on rivers, and almost every country has at least one river through it that plays an important part in the lives of its people.

    Since the beginning of history, people have used rivers for transportation. The longest one in the United States is the Mississippi. The lifeline of Egypt is the Nile. To the people of India, the Ganges is great, but it is also important for transportation. Ships can travel along it for a thousand miles. Other great rivers are the Congo in Africa and the Mekong in southeastern Asia. The greatest of all for navigation(航海), however, is the Amazon in Brazil. It is so wide and so deep that large ships can go about two thousand miles up it.

    Besides transportation, rivers give food, water to drink, water for crops, and chances for fun and recreation for the people who live along their banks. In order to increase the supply of water for crops, engineers sometimes build a dam(坝) across a river and let a lake form behind the dam. Then people can use the water not only to irrigate(灌溉) their fields but also to make electricity for their homes and industries.

    However, large cities and industries that are located on rivers often cause problems. As the cities grow in size and industries increase, the water in the rivers becomes polluted with chemicals and other materials. People are learning the importance, however, of doing more to keep their rivers clean if they want to enjoy the benefits of them.

    The expression "lanshouxianggu" has been quite popular on Chinese social networking sites. You may have used it many times when chatting online. However, did you know that the "blue thin mushroom" was not just magic of Photoshop and was really there in nature?

    Well, let' s meet Entoloma hochstetteri (霍氏粉褶菌).This is a kind of naturally-blue fungus(真菌) The plant looks very small. Its sten' s diameter(梗的直径) measures only about 5 millimeters. Entoloma hochstetteri can be found widely in New Zealand, especially on the west coast. The Maori name for it is "werewere-kokako." It seems New zealand is very proud of producing this unusual species-the country prints the blue mushroom on its $50 notes.

    Athough the species is very unusual, little is known about it. Silas villas-boas is a scientist at the University of Auckland. He said he was very disappointed that nobody had ever tried to test the blue mushroom in the lab, "You can even count the number of scientific articles on it on one hand, he said. The scientist decided to be the first to study the plant. A group of researchers have been helping him with the study.

    In an early test, the researchers found alkaloids(生物碱) and Omega-3 fatty acids(w-3脂肪酸)in the blue mushroom. While the acids are helpful to health, alkaloids soetimes influence how the brain works. However, Villas-Boas said the substance was very common in plants. So far, the researchers have not found any toxins(毒素) in Entoloma hochstetteri, but they are not yet sure if it can be eaten. Villas-Boas said more testing would be required.

    How do animals communicate (交流) with each other? They have special "languages". In fact, they are using signals (信号). Different signals have different meanings.

    For example, when a bee (蜜蜂) finds food somewhere, it flies home quickly. It can't speak, but it can tell others by dancing. Other bees will know where and how far the food is.

    Some animals show their feelings by making sounds. For example, when a dog is angry, it barks (吠). At this time, you'd better stay away from it. Birds can make several sounds to show different meanings. Sometimes we humans show feelings by making sounds instead of by speaking. For example, when we feel painful (疼痛的), we cry "Oh!" or“Ouch!".

    We humans have languages and words. We can communicate face to face by speaking. We can write down what happened in order to remember clearly. We can also send (寄) messages to people far away. But animals can't.