In many high schools, there are strict rules about using mobile phones. If your phone rings in class, for example, the teacher will be very angry.

    Every mobile phone has a ringtone (铃声). It tells you that someone is calling you. Now, there is a ringtone called “mosquitotone (蚊子铃声)”.

    Mosquitotone has a special sound. Only children and teenagers can hear it. As we get older, we are unable to hear some sounds with high frequencies (频率). As a result, a child or teenager can hear the mosquitotone ringtone, but someone of 20 or older can't.

    But sometimes, people over 20 can hear the ringtone. One British student's phone uses mosquitotone. In class, the student's phone rang. The 22-year-old teacher looked at the student and became unhappy. “I heard that. You know the rule：no mobile phone in class. ”

    Not all the students are interested in mosquitotone. Jerry Jones, a high school student of 17, says, “Using mosquitotone too often is bad for our hearing. I may use it for pleasure, but not in class. It's unfair(不公平的) to the teachers！”

    What do parents think of mosquitotone？“I think it is perfect for places like movie theaters,” says Connie Neeson, 44. “Think about it. You're watching a film and someone's mobile phone rings. Mosquitotone is good because many people don't hear it. However, it also has a bad influence（影响） on their study if students use it in class. ”

    Building an app is a big project. If you want to build the next great smartphone app, you need a plan. First, think about the kind of app that you want to build. Will your app be a game? Will it be for business? Then make a schedule(日程安排)for building the app. You need plenty of time for research before you are ready to build.

    Now you're ready to make the app. Building an app is like a puzzle(拼图)with many different parts. You can build it yourself or find a company to help you. There are many videos and websites that can teach you to build one. Some websites provide "building blocks" for app developers. As you build, you pick blocks from the website and put them in your app.

    After you build your app, you need to check it carefully. Look for bugs. Most apps have several problems to fix. Show your app to people. Ask their opinions. Mike sure everything works before letting people download it. There are so many tools that can help you build apps. Having the right tools will make the work easier. Now almost anyone can make a new app.

    Is there intelligent(有智力的) life on other planets(行星)? For years, scientists said "No." or "We don't know." But today this is changing. Seth Shostak and Alexandra Barnett are astronomers (天文学家). They believe intelligent life exists somewhere in the universe. They also think we will soon contact these beings(人；生物).

    Why do Shostak and Barnett think intelligent life exists on other planets? The first reason is time. Scientists believe the universe is about 12 billion years old. "This is long enough for other planets to have intelligent life," say Shostak and Barnett. The second reason is size——the universe is huge. "Tools like the Hubble Telescope (哈勃望远镜) have shown that there are at least 100 billion galaxies (银河系) ," says Shostak. "And our galaxy, the Milky Way, has at least 100 billion stars. Some planets going around these stars might be similar to Earth. "

    In the past, it was hard to look for signs of intelligent life in the universe. But now, powerful telescopes (高倍望远镜) allow scientists to discover smaller planets--the size of Mars or Earth--in other solar systems (太阳系).These planets might have intelligent life.

    Have beings from space already visited Earth? "Probably not," says Shostak. "It's a long way away. However, intelligent beings may contact us in other way, such as radio signals(信号). "In fact, they may be trying to communicate with us now, but we don't have the right tools to receive their messages. However, this is changing. By 2025, we could make contact with other life forms in our universe and we might help each other.