Have you ever rubbed your hands together when they get cold? Why do we do that? We do that for friction. Friction happens when two things that aren't very smooth rub together to make heat energy. The faster you rub two things together, the more friction (and heat) you can make. Let's discover friction!

How does a young dog or cat get to know a new home? The animal uses its nose. Right away it smells its new places. Then it makes wider and wider circles, smelling all the time. Before long it can find its way home very well, even in the dark. It simply follows something it knows well.

    Stories happen about animals who found their way across land they had never smelled before. Take the case of Smoky, a small white cat. Smoky had some funny red hair around its right ear. One day Smoky and his owner began a long journey. They were moving from Oklahoma to Tennessee. When they were just eighteen miles from their Oklahoma home, Smoky jumped out of the car. Then he found his way back to the old house. There he wandered around outside for many days. Finally he disappeared.

    A year later Smoky meowed at the door of a house in Tennessee. A man opened the door. "Is that you, Smoky?" he said in surprise. At first he couldn't believe it. Then he saw the red hair around its right ear. It was Smoky!

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.

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    When you build a house，you use a blueprint．What's a blueprint?In general，a blueprint is a drawing that you use to build a house and shows where to put all the parts of the house．DNA is a drawing,too．It's the blueprint of life．

    But building a living thing is far more complex*than building a house．It takes millions of years for nature to change the DNA in living things to make them fit the world they live in．

    How tall are you?What color are your eyes?What size are your feet?All these things are strongly influenced by your DNA before you are born．

DNA is fantastic!

  ★Except for some twins，every living thing has its own unique*DNA．

  ★Some health problems can be caused by problems in your DNA．Scientists can look for these problems and，sometimes，help you if you become sick．In the future，scientists might be able to change your DNA．   

  ★There are copies of your DNA in every part of your body．For example，your finger has the same DNA as your hair．

  ★You have your own unique DNA，which repeats patterns from your parents' DNA．but not exactly the same．From your DNA，scientists can identify not only you，but also people who are related to you．Your parents and grandparents are related to you，and you have their DNA．When you have a grandson or granddaughter，he or she will have your DNA．

    Your DNA has billions of instructions that explain why you are the way you are．It's one of the most amazing things on the earth !

    Wind power is a very clean source of energy. This is how wind power works. Wind makes windmills spin (旋转). When the windmills spin, they make electricity. Then we can use the electricity.

    A lot of people think that wind power is new, but that's not true.

    For thousands of years, people have used wind to sail boats and move water. We still do those things today, but these days we mostly use wind power to make electricity.windmills

    Wind power is a very popular source of energy. Most people think that we should use it more and more. First of all, it's clean. Windmills don't pollute the environment at all. Second, we can use wind power forever. After all, we will always have wind. Also, wind power is cheap, and it's getting even cheaper.

    A lot of people don't understand wind power very well. They believe that there are problems with wind power, but many of those problems are not real. For example, some people say that windmills are dangerous for birds. They used to be true, but it isn't true anymore. Old windmills killed birds because they spun very fast. New windmills spin slowly, and they aren't dangerous for birds.

    However, there are real problems with windmills. One problem is that many people think windmills are very ugly. This is a problem for people, because windmills are often in very pretty areas. Also, wind doesn't blow all the time, so we can't use wind power all the time. Finally, windmills are noisy.

    Wind power is becoming more and more common. Right now, more than 80 countries use wind power. About 2.5 percent of the world's power comes from the wind. For some countries, that number is a lot higher. Wind power gives Denmark more than 25 percent of its electricity.

    An atlas is-a book of maps. Atlases are made with different kinds of information about different parts and areas of a country or the world. They are prepared for desk use or travel use.

    Desk atlases are made for different groups of people with different needs. For example, students may use desk atlases to help learn Geography, and other people may use desk atlases to study a place. A popular type of the latest atlas shows the recent fact of the world. New atlases are often carefully produced to help people learn about the changes, such as name changes, boundary(界) changes and other important new information. A desk atlas can also have some different maps of the same place. For example, an atlas may include maps showing population and important products of a place.

    Travel atlases usually show the information about both natural and man-made features(特点). So it is not surprising to find universities, airports, forests, rivers, roads as well as cities, towns and villages in them. A travel atlas is often the first thing people need when they want to start a trip in a strange place.

    It often has a map of a whole country, and a map with more special information of each important place in the country. Take the atlas of the United States as an example, it often has a map of the whole country and then a map of each of the fifty states.

    A travel atlas may also point out the beautiful natural places. In the USA, the atlas may include national parks such as the Yellowstone National Park and some others to show the beauty of nature and interesting places to tourists.