Mark Twain, the famous American writer and a great master of humor, liked to play jokes on others. But once a joke was played on him. One day Mark Twain was invited to give a talk in a small town. At lunch he met a young man, one of his friends.

    The young man said that he had an uncle with him. He told Mark Twain that his uncle never laughed or smiled, and that nobody and nothing was able to make his uncle smile or laugh.

    "You bring your uncle to my talk this evening," said Mark Twain. "I'm sure I can make him laugh."

    That evening the young man and his uncle sat in the front. Mark Twain began to speak. He told several funny stories and made everyone in the room laugh. But the man never even smiled. Mark Twain told more funny stories, but the old man still kept quiet. Mark Twain continued to tell his funny stories. Finally he stopped. He was tired and quite disappointed.

    Some days later, Mark Twain told another friend what had happened." Oh," said his friend, "I know that man. He's been deaf for years."

    I was very disappointed not to be able to go to the concert last Friday. The advertisement in the paper said you could buy tickets at the theatre box any day between 10:00 and 4:00. Since I work from 9:00 to 5:30, the only time I could go to the theatre was during my 45-minute lunch break. Unfortunately, the theatre is on the other side of the town, and the bus service between my office and the theatre is not good. But if you are lucky, you can make the round trip in 45 minutes. Last Monday, I stood at the bus stop for 15 minutes. By the time I saw one come, there was not enough time left to make the trip. So I gave up and went back to the office. The same thing happened on Tuesday, and again next day. On Thursday, my luck changed. I got on a bus right away and arrived at the theatre in 20 minutes. When I got there, however, I found a long line at the box office, and I heard one man say he had been waiting for over an hour. Realizing I would not have enough time to wait in line, I caught the next bus and headed back. By Friday I realized my only hope was to make the trip by taxi. It was expensive, but it would be worth it to hear the concert. The trip by taxi only took 10 minutes, but felt like an hour to me. When I got to the theatre, I was relieved to see that nobody was waiting in line. The reason, however, I quickly discovered, was that they had already sold all the tickets.

    Most people who own iPhones use them as their alarm clocks―making it very easy to check emails one last time before falling asleep and hard to ever feel away from work and social networks.

    Several years ago, my boss fainted due to exhaustion after staying up late to catch up on work. She banged her head and ended up with five stitches (缝针) and became what she calls a "sleep evangelist (传教士)." Now she leaves her phone charging in another room when she goes to bed and encourages her friends to do the same.

    "I sent all my friends the same Christmas gift―a lovely alarm clock―so they could stop using the excuse that they needed their very attractive iPhones by their beds to wake them up in the morning." she said.

    If your phone wakes you up in the morning, it may also be keeping you up at night. A 2008 study showed that people exposed to mobile radiation took longer to fall asleep and spent less time in deep sleep. "The study indicates that during laboratory exposure to 884 MHz wireless signals, components of sleep believed to be important for recovery from daily wear and tear are severely affected," the study concluded.

    A quarter of young people feel like they must be available by phone around the clock, according to a Swedish study that linked heavy cellphone use to sleeping problems, stress and depression. Unreturned messages carry more guilt when the technology to deal with them lies at our fingertips. Some teens even return text messages at midnight.

    Most of us choose not to set limits on our nighttime availability. Nearly three quarters of people from the age of 18 to 44 sleep with their phones within reach, according to a 2012 Time poll. That number falls off slightly in middle age, but only people aged 65 and older are leaving the phone in another room as common as sleeping right next to it.

    Whether you're on a long road trip or stuck in traffic while driving home from work, a low phone battery (电池) could mean disaster for the bored driver. At first, it may seem harmless to plug your phone into your car's USB port. But unless you're desperate, charging your phone in your car might be a big mistake.

    Why? First, the USB port in your vehicle probably provides less electricity than your phone really needs to charge. As a result, your phone might stop working while it charges, or worse—hardly charge at all.

    Brad Nichols, a technician at Staymobile, told Reader's Digest. “This is mostly due to the fact the phone is using more power than the car charger is supplying it.”

    Nichols also says that your phone could receive too much power, especially if you're using a “Cigarette Lighter” port to charge up. Most Cigarette Lighters can supply up to 10 amps (安培), while most chargers use one to three amps. A damaged charger can provide inconsistent power to the phone, leading to sudden power increase that could cause damage to the inside parts, or on the rare occasion, destroy it.

    Charging your phone while on the road could use up your car's battery, too. If your engine is off, but you still use the radio—the phone will draw power from your car's battery as it charges. This usually isn't a big deal for those who own new cars with healthy batteries Nichols says. But if your car is an older model, you might want to avoid charging your phone through its USB port.

    Most importantly, it, s not safe to use your phone while operating a vehicle. “Anytime a person's hands leave the wheel or eyes leave the road, it becomes very dangerous for them and the other people around them,” Nichols says. Bottom line: Play it safe, and wait until you get home to plug in.

    Throughout much of human history, man has been the measure of many, if not all, things. Lengths were divided up into feet and smaller units from the human hand. Other measures were equally characteristic. Mediterranean traders for centuries used the weight of grains of wheat to define (定义) their units of mass. The Romans used libra, forerunner of the pound, by referring to the weight of a carob (角豆树) seed.

    The sizes of similarly named units could also differ. The king's foot, used in France for nearly

    1,000 years after its introduction by Charlemagne in around 790 AD, was, at 32.5cm,around a centimeter shorter than the Belgic foot, used in England until 1300.Greek,Egyptian and Babylonian versions of water in a fixed container varied from one another by a few kilos, Nor was there agreement on such things within countries. In France, where there was no unified (统一的) measurement system at the national level, the situation was particularly terrible. The lieue (former measure of distance), for example, varied from just over 3 km in the north to nearly 6 km in the south.

    Although John Wilkins, an Englishman, first put forward a decimal system (十进制) of measurement in 1668,it was the French who in 1799 made it law. The Système International d'Unités (SI, or the metric system, as it is better known) developed from it and became the official measurement in all countries except Myanmar, Liberia and the United States. Now the International Bureau of Weights and Measures in Paris is set to give the metric system its biggest shake-up yet.

    At a meeting in Versailles, France, on November 16th,2018,the world's measurement bodies are almost certain to approve a decision that will mean four out of the seven base SI units, including the kilogram, will follow the other three, including the metre, in being redefined in terms of the values of physical constants (物理常数).Each of the chosen constants has been measured incredibly precisely, which would mean that from May 20^th 2019 the constants will themselves be fixed at their current values for ever. Any laboratory in the world will then be able to measure, for example, the mass of an object as precisely as the accuracy of their equipment will allow.