Our sun is a star, and nine worlds (天体) move round it. Worlds like these are planets. But here are other stars, other suns, not only thousands of them, but millions of them. Do different stars have planets? We can see the planets round our own sun. But we haven't been anymore. The stars are too far away from us. Stars give out light, but the planets don't have any light of their own. So we can't see them. They are too far, and too dark. Still, we can tell that some stars have planets. While a big planet goes round, it pulls its star to one side. And sometimes we can see this. Then we know that the star has a planet. For example, Barnard's Star is a small red star. And it is the quickest star in the universe. But sometimes it moves slower, and then quicker again. We have known Bernard''s Star for many years, and now we are sure it has a planet.

    The world's forests become smaller and smaller. People are still cutting down the trees. Why is this happening? There are two reasons—land and wood. People need land for animals and crops. And people want wood. Japan, Europe and North American need wood. There, it is used for buildings or it is made into furniture.

    But most places from the forests are not good for animals or crops. The soil (土壤) in the forests is very thin. When trees are cut down, the soil is washed away by the rain or the wind. Soon there is nothing but sand. Then more land will be needed and more trees will be cut down.

    One day, you open the fridge and find some moldy (发霉的)food. But before you can throw it away, your mother stops you. She doesn't want to waste food. She cuts off the moldy part and eats the rest. Is it safe to do this?

    Mold (霉菌)can grow on almost all kinds of food. Mold is usually green, white, black or grey. Mold makes food soft and changes its color. Moldy food tastes quite bad一一and it's not safe to eat.

    If you find mold in soft food, just throw it away. Soft food, such as bread, tomatoes or hot dogs, usually contains water. This allows mold to easily grow below its surface. Bacteria (细菌)can also grow along with it.

    But it's easy to remove mold from hard food. The mold can't grow very deeply inside this kind of food. According to the BBC, you should cut about 2.5 cm around and below the mold to remove it. It's OK to cut the moldy parts out of hard food and eat the rest. But if the food is completely covered with mold, you should still throw it away.

    Most kinds of mold are bad for us. But some kinds of mold can be used to make food. For example, penicillium (青霉菌)can be used to make many types of cheeses, such as blue cheese. These cheeses are safe to eat--and they're quite delicious as well.

    You don't have to be a scientist to solve big environmental problems. One Dutch designer is finding creative ways to make a change—and he's inviting bikers to join him.

    Daan Roosgaarde, 38, is a designer from Holland. He uses creative ideas to improve city life. For his Smog (雾霾) Free Project, he designed a special tower that cleans air in Chinese parks. In Holland, he designed a shining bicycle path.

    Now Daan and his team are working with engineers at Chinese bike-sharing company Ofo to create a new product for public use. It's an air-cleaning machine connected to a bicycle and is part of the Smog Free Project. In smoggy weather, the machine would breathe in the air in front of the bike and blow out a stream of clean air toward the biker's face.

    It's no secret that air pollution is a big problem for China. In winter, it's common for thick clouds of haze (霾), caused largely by factory production, burning of coal and car emissions (排放物), to cover areas of China.

    In 2016, only one-fourth of Chinese cities met the country's air quality standards, according to a survey of 338 cities nationwide from the Ministry of Environmental Protection.

    "Of course one tower and a few bikes won't solve China's air pollution problem," Daan said, "But smog-free bikes are an exciting idea that will hopefully inspire communities to create greener cities."