The earth is about 4,600 million years old. Modern man has lived on the earth for only 35,000 years, but during that time, we have changed our planet in many ways. Many of the things that we have done are good, but more are not good for the earth.

    Water pollution

    A lot of people, birds and fish die each day because of water pollution. Factories have polluted the land and the water. As a result, many river and lakes are now dead.

    Air pollution

    In big cities, factories, as well as cars, trucks and buses are the biggest cause of air pollution. Many people in cities now have bad health problems. For example, Mexico city used to be a beautiful sunny capital, but today it is almost covered by thick brown clouds.

    Soil pollution

    In order to have a good harvest, most of the farmers use chemicals too much on their fields. That's bad for the soil. But this kind of pollution is difficult to stop.

    Hope for the future

    These problems are very serious for our future, so all of us should do something to improve our environment. If we can stop pollution, our planet will become more and more beautiful and our health will be better. Let's be greener people.

    Building an app is a big project. If you want to build the next great smartphone app, you need a plan. First, think about the kind of app that you want to build. Will your app be a game? Will it be for business? Then make a schedule(日程安排)for building the app. You need plenty of time for research before you are ready to build.

    Now you're ready to make the app. Building an app is like a puzzle(拼图)with many different parts. You can build it yourself or find a company to help you. There are many videos and websites that can teach you to build one. Some websites provide "building blocks" for app developers. As you build, you pick blocks from the website and put them in your app.

    After you build your app, you need to check it carefully. Look for bugs. Most apps have several problems to fix. Show your app to people. Ask their opinions. Mike sure everything works before letting people download it. There are so many tools that can help you build apps. Having the right tools will make the work easier. Now almost anyone can make a new app.

    SoFi is a special "fish". It is 47 centimeters long. It is all white. It has a small round head. What kind of fish is it? Actually, SoFi is a soft robotic fish. Scientists from MIT made it look like a real fish. They want to use it to study sea animals when the ocean environment changes.

    SoFi's skin is made of a kind of rubber. It's soft. The robotic fish's head is 3D printed. There is a camera in its nose and it can take photos and videos underwater. SoFi can swim among real fish. It is quiet. It doesn't scare other fish. It can also turn left and right, and even change its speed.

    Although SoFi is not the first lifelike robotic fish, it is the most advanced of its kind, according to MIT scientists. Many other countries have also made robotic fish, including the UK and China. Scientists said these robotic fish can easily get close to other fish. And other fish can even interact (互动) with them.

How honey is made

    Many people know that rubbish is a big problem on planet Earth. What many people don't know is that junk has become a problem in outer space too.

    According to BBC News, there are more than 22,000 pieces of space junk floating around the earth. And these are just the things that we can see from the surface of the earth by telescopes (望远镜). There are also millions of smaller pieces of junk that we can't see.

    Objects, like bits of old space rockets or satellites, move around the planet at very high speeds, so fast that even a very small piece can break important satellites or become dangerous to astronauts. If the smallest piece of junk crashed into a spaceship, it could damage it.

    To make things worse, when two objects in space crash, they break into many smaller pieces. For example, when a U.S. satellite hit an old Russian rocket in 2009, it broke into more than 2,000 pieces, increasing the amount of space junk.

    To reduce additional space junk, countries have agreed that all new space tools can only stay in space for 25 years at most. Each tool must be built to fall safely into the earth's atmosphere after that time. In the upper parts of the atmosphere, it will burn up.

    Many scientists are also suggesting different ways to clean up space junk. In England scientists are testing a metal net that can be fired into space junk. The net catches the junk and then pulls it into the earth's atmosphere to burn up. The Germans are building robots that can collect pieces of space junk and bring them back to Earth to be safely destroyed.

    "The problem is becoming more challenging because we're sending more objects into space to help people use their mobile phones and computers," says Marco Castronuovo, an Italian space researcher.

    "The time to act is now. The longer we leave the problem, the bigger it will become," he says.