Most people agree that trees look nice, but not everyone knows that trees can also help cities save money. A few years ago, the US Forest Service studied trees in Chicago. They found that just one tree can save a city hundreds of dollars over its lifetime.

    Trees can save money by helping people save energy(能源). Trees stop the sun shining on buildings. They also block(挡住) the winter wind. That helps people use less energy to make their homes or offices warm or cool. Saving energy is saving money.

    Trees have another way to help a city save money. When it rains, there will be a lot of water in a city's sewers (下水道). Cities need lots of money to clean that water. But before the rainwater gets to the dirty sewers, trees can take some into themselves. With the help of trees, there is less water to clean.

    Many people are trying to save the earth with special days like "Meatout Day", "Earth Day" and "Buy Nothing Day".

    "Meatout Day" is on March 20. Many people around the world don't eat any meat for one day. In this way, they'll get healthier and reduce greenhouse gases (减少温室气体).

    "Earth Day" is on April 22nd. Many people all over the world try to do something green on this day. In Italy, people don't use plastic bags. In Canada and Norway, people ride bicycles and don't use their cars. In the USA, people dress up like plants and animals to dance in Times Square, New York.

    "Buy Nothing Day" is usually on the fourth Friday in November. On this day many people don't buy anything. "Buy nothing" means "make no rubbish". When you make less rubbish, you can make a cleaner earth.

    Do you want to join them? Then you can remember these special days and do something green.

    Autumn in Tibet is very beautiful because there are many fresh and bright colours everywhere.

    In north Tibet, one can travel several hundred kilometres by car and cannot see even a person. No one can be found in the north of Shuanghu-only treeless fields and grasslands. Warm autumn days bring life to all of nature. Wild animals become active, and Tibetan antelopes like to play near the lakes. When they run, sometimes they can reach 80 km an hour, leaving the hungry wolves far behind.

    Autumn in south Tibet is very different from that in north Tibet. Mount Qomolangma in southwest Tibet, the highest mountain on Earth, is world-famous for its beauty. It is over 8, 000metres high. There are over 2,300 kinds of plants and about 300 different kinds of wild animals on Mount Qomolangma.

    On April 2, we said goodbye to Tiangong I, China's first space lab. According to the China Manned Space Agency (中国载人航天), Tiangong I re-entered the Earth's atmosphere (大气层) and some of its debris (碎片) fell into the South Pacific Ocean.

    There are many spacecraft (宇宙飞船) that are still in orbit

     (轨道) above the Earth. After finishing their trips, they will all re-enter the Earth's atmosphere like Tiangong I.

    There are two types of re-entries: controlled re-entry and uncontrolled re-entry.

    Some satellites and manned spacecraft come back to the Earth in a controlled re-entry. Experts calculate (计算) the path of the falling spacecraft and its speed. Thy can guide the spacecraft to fall in a chosen area.

    Some spacecraft may have problems while in space after a certain amount of time. These spacecraft come back in an uncontrolled re-entry. It is hard to tell when and where these spacecraft will fall until the last few hours. The US space station Skylab came back partially uncontrolled in 1979. Parts of the station fell in western Australia, but no one was hurt.

    During re-entry, most of the spacecraft will burn up while passing through the Earth's atmosphere. Only a small amount of the debris will reach the ground. The debris typically ends up falling into the ocean, China Daily reported.