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    Have you ever had dreams about building a robot? Or putting together a spaceship? Or even seeing your face made out of chocolate? Done. All you need is a 3-D printer.

    The possibilities of 3-D printing seem endless(无穷尽的）.All the user has to do is to design an object on the computer and choose a material to print in. The printers print in hundreds of different materials including glass, sugar and even skin! When the user presses(按）“print", the 3-D printer spreads the materials layer(层）by layer and like magic the object is produced.

    The first 3-D printer was invented in 1986 by an American, Charles Hull. But 3-D printing has only become cheap enough recently for most people to use. Designers now use 3-D printing to create unusual things. The Dutch artist Dirk Vander Kooj prints furniture made from old fridges. And doctors have used 3-D printing to print human body parts! The Telegraph reported in February that scientists from Cornell Medical College even printed a man-made ear.

    But like a lot of new technologies, if 3-D printing gets into the wrong hands, it can be dangerous.

    In 2011 a group of four men in the US used 3-D printing to produce ATM skimmers(提款机盗刷器).These were placed on an ATM and stole over $400,000 from users of the ATM. It is terrible to think that criminals like them might one day use a 3-D printer to print a gun.

    But for better or for worse, 3-D printing has been put into use. In a few years, you'll probably be wondering how you could ever live without your 3-D printer!

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    You may know about "junk food" like French fries. But do you know about "junk sleep"?

    Recently, a British survey found that electronic products（电子产品）in teenagers' bedrooms are affecting（影响）their sleep. The survey was done among 1,000 British kids from 12 to 16. It found that 30 percent of them got just 4 to 7 hours of sleep every day. But doctors say they need 8 to 9 hours.

    Almost a quarter of the kids said they fell asleep more than once a week while watching TV, listening to music or using other electronic products.

    "This is very worrying." said Dr. Chris Idzikowski, "We call it "junk sleep". It means you don't get enough sleep and the quality（质量）of the sleep is bad, too. If you don't get good rest, you won't perform well in school the next day."

    The survey found that 40 percent of the kids felt tired each day, with girls between 13 and 16 feeling the worst. Nearly all the teenagers have a phone, Mp5 or TV in their bedrooms. And many of them have all three.

    When your pen is broken, the battery in your toy runs out, or you have some leftover food, what do you do with these things? You will probably throw them all into one trashbin. But actually, all of these pieces of trash need to be sorted (分类) separately.

    Garbage sorting is a big problem worldwide. In recent years, some Chinese cities have been working hard on it.

    Shanghai has worked with Alipay to create a "green account (账户)" service for its residents (居民). Account holders get points by correctly sorting their garbage. Through the Alipay app, they can exchange the points for milk, phone cards and other products. The city is asking all of its residents to sort their garbage into four groups: wet, recyclable (可回收的), harmful and dry.

    Wet waste is also known as household waste. "They are things you don't want but that pigs can eat," Guangzhou Daily explained.

    Paper, metal (金属), glass and other things that can be reused are recyclable waste.

    Harmful waste includes things like medicine, batteries and fluorescent bulbs (荧光灯泡).

    Finally, any waste that's not wet, recyclable or harmful will go in the "dry waste" bin.

    Many other Chinese cities are also using this method to sort their garbage. For example, Shenzhen has been sorting its garbage into the same four groups since 2012. Students there also receive waste-sorting guidebooks that they must study.

    China is improving its waste-sorting efforts. There is still a long way to go. But it's never too late to learn how to sort your trash properly and protect the environment.

    If you don't sort your garbage, all of it will go to a landfill (垃圾填埋场) and be buried together. These landfills can take up large areas of ground that could have been used for planting trees. The electronic waste you throw away, such as batteries, can pollute the soil and water. Other pieces of garbage, like the metal part of a pen, can be used to make other things if they are properly recycled.