You may have heard of the American Dream, an ideal that has powered the hopes of Americans for generations.

    It began as a belief that the US was a land of opportunity, and that anyone could achieve success through hard work. The dream has referred to home ownership, a good job, retirement security or each generation doing better than the last for a long time.

    Yet today, this concept seems to have greatly changed. As Tune magazine pointed out, quite different from the older generation, many Millennials (the generation born after 1980) redefine the American Dream as “day-to-day control of your life”. They “prize job mobility, flexible schedules, any work that is more interesting than typing, and the ability to travel”, said the magazine.

    Home ownership, once the cornerstone of the American Dream, is becoming a smaller priority for this generation. Meanwhile, nearly 40% of them choose travel as part of their dream. And running their own business is a rising favorite, as nearly 26% of Millennials consider self-employment as part of their dream.

    So what has led to this huge change?

    Many point fingers at the poor economy. “Modern young Americans seem bound to face a world stamped by ever narrowing opportunity,” noted The Daily Beast.

    “The rate of 16-to-24-year-olds out of school and out of work is unusually high at 15%. Many college graduates have taken jobs that don't require a degree,” Time reported.

    The magazine worries that these difficulties may lead to a lost generation who are “unable to ever truly find their feet on the corporation's ladder”.

    Dan Kadlec, a reporter of Time, sees Millennials as resetting their expectations. “This situation is different for young adults today,” he wrote. “A true American dream has to feel attainable, and many Millennials are feeling they can only attain a day-to-day lifestyle that suits them.”

    Scientists say we are all born with a knack for mathematics. Every time we scan the cafeteria for a table that will fit all of our friends, we're exercising the ancient estimation center in our brain.

    Stanislas Dehaene was the first researcher to show that this part of the brain exists. In 1989, he met Mr. N who had suffered a serious brain injury. Mr. N couldn't recognize the number 5, or add 2 and 2. But he still knew that there are “about 50 minutes” in an hour. Dehaene drew an important conclusion from his case: there must be two separate mathematical areas in our brains. One area is responsible for the math we learn in school, and the other judges approximate amounts.

    So what does the brain's estimation center do for us? Harvard University researcher Elizabeth Spelke has spent a lot of time posing math problems to preschoolers. When he asks 5-year-olds to solve a problem like 21+30, they can't do it. But he has also asked them questions such as, “Sarah has 21 candles and gets 30 more. John has 34 candles. Who has more candles?” It turns out preschoolers are great at solving questions like that. Before they've learned how to do math with numerals and symbols, their brains' approximation centers are already hard at work.

    After we learn symbolic math, do we still have any use for our inborn math sense? Justin Halberda at Johns Hopkins University gave us an answer in his study. He challenged a group of 14-year-olds with an approximation test: The kids stared at a computer screen and saw groups of yellow and blue dots flash by, too quickly to count. Then they had to say whether there had been more blue dots or yellow dots. The researchers found that most were able to answer correctly when there were 25 yellow dots and 10 blue ones. When the groups were closer in size, 11 yellow dots and 10 blue ones, fewer kids answered correctly.

    The big surprise in this study came when the researcher compared the kids' approximation test scores to their scores on standardized math tests. He found that kids who did better on the flashing dot test had better standardized test scores, and vice versa (反之亦然). It seems that, far from being irrelevant, your math sense might predict your ability at formal math.

    Some of the world's most significant problems never hit headlines. One example comes from agriculture. Food riots（暴动）and hunger make news. But the trend lying behind these matters is rarely talked about. This is the decrease in the growth in production of some of the world's major crops. A new study by the University of Minnesota and McGill University in Montreal looks at where, and how far, this decline is occurring.

    The authors take a vast number of data points for the four most important crops: rice, wheat, corn and soyabeans. They find that on between 24% and 39% of all harvested areas, the improvement inproduction that took place before the 1980s slowed down in the 1990s and 2000s.

    There are two worrying features of the slowdown. One is that it has been particularly sharp in the world's most populous countries, India and China. Their ability to feed themselves has been an important source of relative stability both within the countries and on world food markets. That self-sufficiency (自给自足) cannot be taken for granted if productions continue to slow down.

    Second, production growth has been lower in wheat and rice than in corn and soyabeans. This is problematic because wheat and rice are more important as foods, accounting for around half of all calories consumed. Corn and soyabeans are more important as feed grains. The authors note that "we have preferentially focused our crop improvement efforts on feeding animals and cars rather than on crops that feed people and are the basis of food security in much of the world."

    The report also states the more optimistic findings of another new paper which suggests that the world will not have to dig up a lot more land for farming in order to feed 9 billion people in 2050, as the Food and Agriculture Organisation has argued.

    Instead, it says, thanks to slowing population growth, land currently ploughed（犁）up for crops might be able to revert (回返) to forest or wilderness. This could happen. The trouble is that the prediction assumes continued improvements in productions, which may not actually happen.

    Teachers' Day is, interestingly, not celebrated on the same day all over the world. In some countries it is marked by a holiday, in others it is a working day.

    International Teachers' Day is celebrated on October 5. Being held ever since 1994, it is in honor of teachers' organizations worldwide. Its aim is to encourage supports for teachers and to meet the needs of students.

    China   In 1939, Teachers' Day was set for August 27, Confucius' birthday. In1951, the celebration was stopped. However, it was founded again in 1985, and the day was changed to September 10.

    India   Teachers' Day is celebrated on September 5 in honour of Dr. Sarvepalli Radhakrishnan, the second president of India, whose birthday falls on that day.

    Russia   In Russia, Teachers' Day was celebrated on the first Sunday of October between 1965 and 1994. Since 1994, the day is observed on the October 5, just the same day as World Teachers Day.

    The US   In the US, Teachers' Day is non-official holiday. It is celebrated on the Tuesday of the first full week of May. A number of activities are organized in schools to honor teachers. The celebrations can continue for the whole week of May.

    Thailand   In Thailand, National Teachers' Day is celebrated every year on January 16. The first Teachers' Day was held in 1957. The day is a holiday in schools.

    Walk through the Amazon rainforest today and you will find it is steamy, warm, damp and thick. But if you had been around 15,000 years ago, during the last ice age, would it have been the same? For more than 30 years, scientists have been arguing about how rainforests like the Amazon might have reacted to the cold, dry climates of the ice ages, but until now, no one has reached a satisfying answer.

    Rainforests like the Amazon are important for mopping up CO₂ from the atmosphere and helping to slow global warming. Currently the trees in the Amazon take in around 500 million tons of CO₂ each year: equal to the total amount of CO₂ giving off in the UK each year. But how will the Amazon react to future climate change? If it gets drier, will it still survive and continue to draw down CO₂?

    Scientists hope that they will be able to learn in advance how the rainforest will manage in the future by understanding how rainforests reacted to climate change in the past. Unfortunately, getting into the Amazon rainforest and collecting information are very difficult. To study past climate, scientists need to look at fossilized pollen, kept in lake mud. Going back to the last ice age means drilling deep down into lake sediments （沉淀物）which requires specialized equipment and heavy machinery. There are very few roads and paths, or places to land helicopters and aeroplanes. Rivers tend to be the easiest way to enter the forest, but this still leaves vast areas between the rivers completely unsampled（未取样）.So far, only a handful of cores have been drilled that go back to the last ice age and none of them provide enough information to prove how the Amazon rainforest reacts to climate change.

    Nowadays six Amazon Scout delivery robots rolled out in a pilot program in Snohomish County, Wash. The robots carry meals, groceries and packages to homes and offices in this region just north of Seattle. They have appeared on the sidewalks of London, Beijing and other cities and communities worldwide. These machines must overcome pedestrian legs, naughty dogs and broken pavement, which raises some questions.

    These services are gaining attraction as a growing number of city residents expect immediate or scheduled delivery for just about everything. Between 2017 and 2018 online retail sales in the U.S. increased by 16 percent. On the final step of all these deliveries, called the last mile, humans on bicycles, motorized scooters (电动车) or large delivery trucks typically deliver packages. All the vehicles compete for space on busy urban streets. "Deliveries are trending upwards in all crowded city centers, and if city and state leaders don't start thinking about creative solutions like robot deliveries, we can expect even worse traffic jams," says Paul Mackie, director of a transportation policy research center in Arlington.

    A study by this center found 73 percent of delivery vehicles in Arlington were parked outside of authorized areas, often blocking bike lanes and crosswalks. By moving the last step of deliveries from the road to the sidewalk, cities could reduce traffic jams and solve the parking problem entirely, Mackie says.

    Companies such as Amazon are not developing this delivery technology simply to clear up urban traffic. Self-driving vehicles and sidewalk robots could cut down last-mile delivery costs in cities by as much as 40 percent, according to a 2018 report by a consultancy firm. A delivery robot can cost thousands of dollars to manufacture, and most currently require human management and conservation. But in the long run companies that use autonomous delivery vehicles in the next several years could end up saving billions of dollars, the report stated.