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!

    All over the world the need for more laptops and computers increases each year. Networks have been part of our life. Probably everyone with a portable device （移动设备）has once logged on to a public WiFi network without paying for it, while they are having a coffee, on a train, in a shopping mall or at a hotel. However, is this kind of public WiFi network safe?

    Some networks are better protected than others because encryption （加密）methods are used. Therefore, they are safer than others. But a survey in 2015 showed that more than 950 million records were not protected, including addresses, emails, birth dates, phone numbers, passwords and so on. These open WiFi networks are really not safe, because it is very easy for hackers （黑客） to break into the operating system and get people's private information.

    Most smartphones and laptops, by themselves, search and connect to WiFi networks. They usually prefer a network with a connection that has been used before. Hackers' devices are able to record these searches and look like the trusted WiFi networks. When these smartphones and laptops log on to the networks which are not real, hackers can easily steal their private information.

    So next time, when we want to use our laptops or our smartphones in public places which offer free WiFi networks, please think it twice or take safe measures before using them.

根据短文理解，选择正确答案。

    Do you and your friends ever talk about your dreams with each other? If you do, you might have noticed something interesting – some of your friends seldom remember their dreams, but some can always describe their dreams so clearly that it seems like they're describing things that really happened to them. What makes those people different?

    The answer is simple. There are two different types of dreamers – low dream recallers（回忆者）and high dream recallers.

    Low dream recallers usually remember their dreams only twice a month. But high dream recallers are able to remember them about five mornings a week. And a new study suggests that activity in a certain part of the brain could have something to do with it, reported The Huffington Post.

    Perrine Ruby, a French researcher at the Lyon Neuroscience Research Center, studied 41 people (21 high dream recallers and 20 low dream recallers) and recorded their brain activity.

    She found that a part of the brain called the temporo-parietal junction (颞顶联合区) was more active in high dream recallers than in low dream recallers – both when they were sleeping and awake.

    This brain area collects and processes（编程）information from the outside world. This means that high dream recallers know more about what's happening around them. For example, when they are awake, they respond (对……有反应) more strongly to hearing their own names, and when they are sleeping, they are woken more easily by sounds and movements.

    By closely studying people's brain activities, Ruby found that high dream recallers have twice as much “wakefulness time” during sleep as low dream recallers do. And it is during these short times of wakefulness that the brain remembers dreams.

    “The sleeping brain is not able to remember new information,” Ruby told The Washington Post. “It needs to wake up to be able to do that.”

    This is not hard to understand. Just try to think of your own sleeping experiences. If you are worried during the night, you are more likely to remember your dreams, but if you sleep well, you will remember little in the morning, and this is because “you never get a chance to remember”, Robert Stickgold, a Harvard Medical School researcher, told The Washington Post.

    If you do not use your arms or your legs for some time, they will become weak; when you start using them again, they slowly become strong again. Everybody knows that. Yet many people do not seem to know that memory (记忆) works in the same way. When someone says that he has a good memory, he really means that he keeps his memory in practice by using it. When someone else says that his memory is poor, he really means that he does not give it enough chance (机会) to become strong.

    If a friend says that his arms and legs are weak, we know that it is his own fault (过错). But if he tells to us that he has a poor memory, many of us think that his parents may be to blame (责备), and few of us know that it is just his own fault. Have you ever found that some people can't read or write but usually they have better memories? This is because they cannot read or write, they have to remember the things. Since they cannot write them down in a small notebook, they have to remember days, names, songs and stories. Therefore, their memory is being exercised the whole time.

    So if you want to have a good memory, learn from the people：practise remembering things as other people do.

    The Vieira family moved to the United States in 1981. At that time, they decided to stop speaking Spanish at home and only speak English.  This was because they were now living in an English-speaking country.

    Now the Vieira children are adults, and they sometimes travel to Spain to visit their family and friends. But they can't communicate with them. Unfortunately this is common. When people move to new countries, they often leave their own customs and mother language behind. This is a mistake because there are many benefits to knowing more than one language.

    One advantage of speaking more than one language is that you can communicate directly with more people around the world. You do not have to depend on others to talk for you, so you're more independent (独立). People who know only one language must use others to help them communicate.

    Furthermore, speaking a second language also allows people to experience other cultures and customs more effectively. Local expressions, vocabulary, and even jokes can have a powerful effect on a person's understanding of another culture. So, knowing more than one language increases your knowledge of other cultures.

    Finally, having more multi-language speakers improves relationships between countries. If people from different countries can communicate well, the world can become closer. Countries will then have more respect for each other and better understand each other.

    The benefits of knowing more than one language are clear. In fact, there are no disadvantages. It is very sad that many people don't try to learn another language. It is even worse when people let themselves forget their mother language and become silenced.

    We always think that it's OK for us to leave our plants home during our vacation as plants won't scream like pets, right?

    A recent study done by a group of scientists at the Tel Aviv University has discovered that some plants will scream when they are under stress.

    The research was carried out on tomato plants and tobacco plants by cutting their stems (茎) and depriving (剥夺) them of water. A microphone was placed 10cm away from them. When their stems were cut, the plants started "screaming" between 20 and 100 kilohertz, and the scientists guess that this scream is probably meant to warn other plants nearby.

    When the tomato plants' stems were cut, 25 ultrasonic (超声的) pain sounds were recorded in an hour, and at the same time, 15 ultrasonic pain sounds were recorded from the tobacco plants. When they were deprived of water, the tomato plants recorded even more pain sounds with 35 pain sounds in an hour while the tobacco plants recorded about 11 pain sounds.

    The plants also acted differently to different kinds of stresses. For example, the tobacco plants gave out a stronger "scream" when they were deprived of water than when their stems were cut. But plants that were in no immediate (立刻的) danger gave out less than one ultrasonic sound per hour.

    The group of scientists said that plants are not as quiet as we had always thought, but in fact their voices are so high-pitched that we just can't hear them.

    Last year, another study has found out that plants feel pain when they are touched or hurt and they will give off a terrible smell.