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.

    Do you know the whales in the sea? There are many relatives(亲属) in the whale family and they always live together with whales.

    Few people know dolphins(海豚) are part of the whale family. Dolphins are not fish. They are mammals(哺乳动物). They breathe air, and they have babies and feed them with milk. Dolphins are famous for their wonderful shows. They can jump high out of the water and show turns in the air. These jumps give the dolphins time to breathe.

    Porpoises(鼠海豚) are also in the whale family and are quite like dolphins. But they are different in the size and shape of the mouth. The dolphin's mouth is long and thin, but the porpoise's is short and thick. Dolphins are also more talkative than porpoises. Dolphins make sounds to talk with each other under water. But porpoises do not do this. Both of them are smart and friendly to humans.

    Not all the members of the whale family are friendly. Big whales are much easier to be angry than smaller dolphins or porpoises. An angry whale can be dangerous. Perhaps this helped to create the story of Moby Dick that sank (使下沉) a ship and made all the people on it die in the sea.

    Space vegetables are grown from seeds(种子) that have been taken to space and brought back to the earth. The seeds are affected by the radiation(辐射)and low gravity in space. When they are brought back to the earth, these seeds produce vegetables that are bigger and healthier than normal vegetables. However, some people worry about eating space vegetables. They think that space vegetables might not be good for us and could make us get sick because of the radiation in space. However, people should not be frightened because space vegetables are very healthy.

    Here are some facts that you should know about space vegetables.

Space vegetables are grown from seeds that are carefully chosen. When seeds are brought back from space they are tested to make sure that they will be safe to eat.

    Space vegetables are better for you than normal vegetables. For example, space tomatoes stay fresh for twenty days, which is one week longer than normal tomatoes。

    After genetically modified (转基因)food appeared in the market, people worried that they were eating unknown things. For example, if nut genes(坚果基因) are put inside potatoes, people allergic(过敏的) to nuts might get sick from eating these potatoes because they do not know they are also eating nuts. Unlike genetically modified food, space vegetables have not been genetically changed. This means that no new genes are put into the vegetables. Therefore, there are no dangers of eating unknown.

    People in Western countries are eating less meat these days, whether for their health or because they want to protect animals. But now they have another reason.

    Eating less meat can help to prevent climate change, according to a recent study published in Nature.

    Raising livestock (家畜) is bad for the environment. When animals like cows digest (消化) food, their stomachs make greenhouse gases, which are a major cause of global warming. Seventeen percent of greenhouse gas emissions (排放) come from livestock farming, The New York Times reported.

    In addition, to raise these livestock, people have to cut down forests and turn them into farms. Forests play a key role in fighting global warming because they absorb greenhouse gases.

    Eating less meat according to certain guidelines could cut global food-related emissions by nearly a third by 2050, the study found. Maintaining (保持) a vegetarian diet in the long term could bring down emissions by 63 percent.

    But eating less meat won't be easy. According to the study, people in Western countries will need to eat 90 percent less meat than they do now in order to have an impact (影响) on our climate. Scientists are now trying to find new foods to replace meat.

    Some say that insects (昆虫) could help. A 2013 UN report said that eating insects is healthier and better for the environment. They produce 75 percent less greenhouse gases than livestock.

    Some experts are creating lab-grown meat. They make meat in a lab using livestock cells (细胞). In March, US company JUST said people in the US might be able to buy its lab-grown meat by the end of this year.