Mexico sites on an island plateau (高原) surrounded by volcanic peaks, which makes air quality a constant concern for people who live there.

    In April, the country took a decisive step toward improving air quality by enacting a temporary ban on private and Federal vehicles in the city. The rule forbids people from driving in the city one day each week and one Saturday each month. Electric vehicles, government service vehicles, public transport options and school buses are not included in the ban.

    Mexico City isn't the first urban center to be involved in car-free living. But the ban is more than an awareness-raiser. It was enacted with the direct aim of solving air pollution. In March, the city sank into a deep brown haze of smog when the pollution levels passed the 200 mark. The city ordered some 1.1m of the area's 4.7m cars off the streets and also offered free bus and subway rides.

    Mexico City's temporary vehicle ban raises questions about the best ways to improve urban air quality. India, whose citizens breathe some of the world's dirtiest air, has tried a variety of solutions. Earlier this year, the Indian government started a 4% sales tax on new-car purchases.

    Beijing also has experience with cleaning the air. The city introduced alternate-day rules in advance of the 2008 Olympic Games, which produced good results. And then there is the London congestion (拥挤) charge, introduced in 2003. The charge has had a measurable effect on air quality.

For now, Mexico City — named by the United Nations as the most polluted city on the planet in 1992 — can look forward to the start of the region's rainy season, when daily showers will help lo clean the air.

    Will it matter if you don't take your breakfast? Recently a test was given in the United States. Those testes included people of different ages, from 12 to 83. During the experiment, these people were given all kinds of breakfast, and sometimes they got no breakfast at all. Special tests were set up to see how well their bodies worked when they had eaten a certain kind of breakfast. The results show that if a person eats a proper breakfast, he or she will work with better effort than if he or she has no breakfast. This fact appears to be especially true if a person works with his brains. If a student eats fruit, eggs, bread and milk before going to school, he will learn more quickly and listen with more attention in class. Contrary to what many people believe, people become so hungry at noon that they eat too much for lunch, and end up gaining weight instead of losing. You will probably lose more weight if you reduce other meals.

    Many Americans buy an evergreen tree for Christmas. They put the tree in their home and hang small lights and colorful things on it. The evergreen is usually a pine or a fir tree (冷杉树). It remains green during the cold, dark months of winter in the northern part of the world. So it is a sign of everlasting life.

    Early long ago, Romans probably planted evergreen trees during a celebration in honor of their god of agriculture. The Christmas tree may have developed from a popular play performed hundreds of years ago in what is now Germany. Traditionally, the play was on December 24th, the day before Christmas. The play was about the first people that God created—Adam and Eve. People put apples on an evergreen tree to represent the tree of knowledge in the Garden of Eden.

    By the year 1600, some Germans began bringing evergreen trees into their homes. They put fruit, nuts and sweets on the trees. They gave the food to the family members and friends after the holiday season. Some people say the German religious reformer (宗教改革者) Martin Luther was the first person to add lighted candles to the tree. They say he did this to show how wonderful the stars had appeared as he travelled one night.

    In the early 1800's, German settlers in the state of Pennsylvania were the first to celebrate the holiday with Christmas trees in the United States. The Christmas tree tradition spread to many parts of the world. Today, some form of the Christmas tree is part of most Christmas celebrations.

    Fashions have a lot of rules. Most of them, however, are just wrong. But there's one rule that goes beyond tradition and into the field of scientific study of the brain: Black garments are slimming. It all comes down to how your visual system processes the light. The below holes in each square are the same in size, yet the white hole looks bigger than the black hole.

    In the 1500s, Galileo Galilei noticed that some of the planets looked larger when viewed with the naked eye (肉眼) than they did when viewed through a telescope, making the white light of Venus (金星) appear eight to ten times larger than Jupiter (木星) in the night sky. He knew something strange must be going on with his vision to cause this illusion, but he wasn't sure what it was. Luckily, scientists never stopped wondering, and in 2014, they figured it out.

    Our visual system operates via two main channels: "on" neurons (神经元) that are sensitive to light things and "off" neurons that are sensitive to dark things. When it came to the dark "off" neurons, the researchers found that they responded predictably to dark shapes on a light background — the greater the contrast between the two, the more active these neurons were. But the light "on" neurons behaved unpredictably. Even with the same amount of contrast, light objects on a dark background caused a greater response in these neurons.

    This makes some sense, evolutionarily speaking. In the dark of night, you'd want to be able to take in every bit of light you can get, so a visual system that enlarges light objects on a dark background could be very useful. However, it's not that hard to see dark objects in the light of day. It has some effects in the colors of your clothes and in the appearance of the planets—the brighter appearance of Venus in the night sky makes it look bigger than the darker Jupiter.

    Bees may soon need to add protein shakes to their diet. Pollen (花粉) normally provides their protein (蛋白质). But rising levels of carbon dioxide (CO₂) in the air have reduced the protein in pollen. That's what a new study finds.

    Scientists compared recently collected flowers from Canada goldenrod. Then they compared pollen in them to pollen in goldenrod reserved at a museum in Washington, DC. They found today's plants have less protein. Protein levels in pollen from the older flowers were 18 percent. Today's goldenrod pollen has only about 12 percent protein. That's about one-third less than 172 years ago. Over that same period, CO₂ levels have gone up by more than one-third. Back more than 170 years ago, they were about 280 parts per million in air. Today they are about 398 ppm.

    To test whether CO₂ changes had played a role in the falling protein levels, the researchers did some tests. They grew goldenrod for two years at CO₂ levels of up to 500 ppm. More CO₂ in the air led to lower levels of protein in the flowers' pollen.

    Joan Edwards, one of the researchers, says that this drop in pollen protein could affect a bee's diet and health. It would be like eating junk food, tasting good but offering less nutrition (营养).

    Honeybees need to eat lots of protein. They use it to feed their young. They also need it to keep their immune systems healthy, says Cédric Alaux, a bee biologist at the French agricultural research agency.

    Bee populations have been decreasing worldwide in recent years. A drop in their food quality might play a role, Edwards says. And that drop might affect crops too. "The health of the bee population is not just for the flowers and the bees and biodiversity." She worries it also could play a role in "human health and well-being."

    Amazon.com has received a patent (专利) right to send its products to customers by drone planes. The company is the largest US-based online seller of products. It delivers those products to customers by mail and other services. Now, the company has received a patent for its planned drone delivery system.

    In March, federal officials allowed Amazon to test its drone delivery plan. But even with a patent and the ability of drones to make deliveries, it does not appear that the company will put its new delivery system in place any time soon.

    The US Patent Office published the document. It describes Amazon's plans to use GPS to deliver products to homes and to wherever people are. The drone will use a customer's mobile device—usually a smartphone—to avoid hitting people or buildings and to find a secure area to land. The company wants to deliver products within 30 minutes after people ordering them on its website.

    However, the Federal Aviation Administration (FAA) is worried about the safety. Although it allowed Amazon to test the system, the agency said the operators must be able to see the planes. That greatly limits (限制) the delivery area.

    Loretta Alkalay is a professor at the Vaughn College of Aeronautics and Technology in New York. Although Amazon is not yet offering drone delivery, Professor Alkalay says it will not be long before it does. She says there are worries that the drones will hit buildings or people, or that the products and drones will be stolen or damaged by people on purpose. But she says that is because many people believe the drones would only deliver the products to the front door of a house. "However, it is possible that homes will one day have special areas on their roofs designed to receive product deliveries," she adds.