Have you ever experienced the feeling of falling as you start to fall asleep? Sometimes you may come to realize this feeling. If so, don't worry. Many people have felt the same thing.

In fact, these muscle twitches (肌肉抽搐) in the arms, legs or other parts of the body are called hypnic jerks (入睡抽动) or sleep starts. And they are very common. Up to 70 percent of people experience them sometimes, Huffington Post reported. But what causes it?

There are some theories(理论). Some scientists believe that the feeling of falling comes from the body relaxing itself, especially if the person is anxious or unable to get comfortable. As you're about to sleep, your muscles become relaxed. But your brain stays awake. It misunderstands the relaxing of your muscles as a sign that you're falling. Then the brain jerks the body awake as if it's trying to protect you.

    Other scientists believe that different phases of sleep might cause the hypnic jerks. When we sleep, we experience two phases: non-rapid eye movement (NREM) and rapid eye movement (REM). During REM, our heart rate (心率), breathing and nervous system can act irregularly (不规律地). If the body experiences flashes (一瞬间) of REM while entering sleep, these irregularities could cause twitches.

If you don't want to experience hypnic jerks, scientists have some suggestions. When you become deeply relaxed, you can learn how to reconnect slowly by moving your fingers and toes a little bit. Then start to move your head and open your eyes, while being aware of yourself as you get out of the relaxed state.

    But experts agree that hypnic jerks, though sometimes scary, are harmless. If it wakes you up, simply roll over and go back to sleep. But keep in mind: a caffeinated (含咖啡因的) drink, exciting evening activities or stress may make hypnic jerks happen more often.

    Have you ever seen a car without a driver? It sounds crazy, but these computer-driven driverless cars will soon be filling roads near you. Companies like Google and Tesla have been designing and testing these cars, and the technology is there.

    So how? The cars have sensors（感应器）all around which can find other cars and objects in the road. Road signs are read by cameras, and satellite navigation systems（卫星导航系统）are used so the car knows how to get where you want to go. All you have to do is type in the address! Finally, a central computer system takes in all the information that it receives from the sensors and cameras and works out when to speed up, stop and turn.

    Sound like your idea of heaven（天堂）? Sitting back, looking out of the windows and even watching a film or reading a book while ‘driving' would be possible with this new technology. You wouldn't have to worry about remembering directions to where you're going. In addition, computers would also drive more safely than people — they would obey the rules and have quicker action times when in dangerous situations.

However, there are many drawbacks of driverless cars. Computers would have difficulties making ethical（道德的）decisions: if a child ran into the road, would the computer choose to hit the child or turn suddenly and potentially kill the car's passengers? Moreover, I personally find driving fun –I'd miss never driving a car myself. There would also be many other decisions to be made –should children, or drunk people, be allowed in a driverless car by themselves? Or would there need to be a person with a driving license in the car at all times?

    I'm not certain I'd want a driverless car – but it's only a matter of time before they'll become more affordable and popular on our roads.

    In most parts of the world, many students help their schools make less pollution．They join environment clubs．In an environment club, people work together to make our environment clean．

    Here are some things students often do．

    No-garbage lunches．How many lunch bags do you throw away after lunch? Environment clubs ask students to bring their lunches in bags that can be used again．Every week they will choose the classes that make the least garbage and report them to the whole school．

    No-car day．On a no-car day, nobody comes to school in a car including the students and the teachers．Cars give pollution to us, so remember:

    Walk, jump, bike, or run!

    Use your legs! It's lots of fun!

    Turn off the water! Do you know that some toilets can waste twenty to forty cubic meters of water an hour? In a year, that would fill in a small river. In the environment clubs, students mend those broken toilets．

    We love our environment．Let's work together to make it better．

    In the universe, as we all know, there is the Sun, the Moon, the Earth and a good many stars. Through our geography lessons, we know the Earth goes around the Sun, and the Moon goes around the Earth. We have day and night because the Earth keeps turning all the time. When our part of the Earth turns to the Sun, it is day. When our part of the Earth turns away from the Sun, it is night.

    Just because the Moon is closer to the Earth than the Sun, it looks much bigger than the Sun. Big things will look smaller when they are further, and small things also look bigger when they are nearer.

    The Sun is big enough to give out very strong light. The Moon can't give out any light at all, but it looks quite bright, too. Why? In fact, the light from the Moon comes from the Sun. The Moon looks much bigger and brighter than the stars. But actually the stars are much bigger and brighter than the Moon. They look smaller than the Moon also because they are farther away from the Earth.

    There are many other planets in the universe. But of all the planets, only on the Earth there are living things and people can only live on the Earth, too. But at 9: 00 a. m. on October 15th, 2003, Chinese pilot Yang Liwei was sent up to space. He stayed there for over 21 hours. It was the first time for Chinese to get into space.

    There are some easy things you can do to protect the environment and the earth. Choose ideas from the list or come up with a few of your own.

    •Plant flowers, grass or trees.

    •Whenever you visit a park or beach, take away what you bring there—keep rubbish in a bag until you can put it in a dustbin.

    •Turn off the lights and TV sets when you leave the room. This can save a lot of electricity.

    •Turn off the tap when you brush your teeth. You can save some water by not letting it run. Also, use a glass cup instead of a paper cup because this saves paper.

    •Keep the doors and windows closed in winter to keep warm air in.

    •Give your old books and magazines to a library instead of throwing them away.

    •Give your old clothes to poor children you know instead of throwing them away.

    •Use both sides of paper.

    •Stop pouring dirty water into the rivers or lakes nearby.

    •Encourage all your friends to do the same things you do to help protect the earth. You don't have to wait until Earth Day to do these things. Make every day Earth Day. If everyone makes a contribution to protecting the environment, the world will become much more beautiful.

    Imagine one day, the water taps (水龙头) in your house stop running. You have to buy water from shops. And still there isn't enough for everyone. Your mother has to save the family's shower water to do other things. Would you be able to stand that kind of life?

    Probably not. But that's what kids in Yemen are experiencing. Experts said Yemen is going to be the first country in the world to run out of water. According to a report, the capital, Sana, will run out of drinking water as early as 2025.

    Because of the shortage, the government often cuts the water supply(供应). Hannan, an 18-year-old from Lahej, told Times, "In a good week we'll have a water supply all week. But then the following week there will be water only for a day or two."

    Hannan said only rich people have enough water to use. They can buy water from the shops or from the water trucks. Private companies own the trucks. They travel around the city every day to sell water—at very high prices. "A lot of people can't afford it," she said.

    The average(平均) person in Yemen uses 100 to 200 cubic(立方) meters of water per year. That is far below the international water poverty(贫困) line of 1,000 cubic meters.

    The government is thinking of making use of seawater. But it will cost a lot and it may not happen soon enough to help the people of Yemen.