If you see a group of people dancing and singing on the street or in the railway station,  you don't need to feel surprised．They are a flash mob （快闪族）, which is a group of people who come together suddenly in a public place, do something unusual for a brief period of time,  and then quickly break up．They are usually organized with the help of the Internet or other digital communication network．At a predetermined time, they gather and perform some  distractions（消遣） such as waving their hands and exchanging books, Then, they quickly break up before the police can arrive．Using mobile phones, the flash mob can change its location if the first one has been replaced for any reason．

    Bill Lasik, senior editor of Harper's Magazine, organized the first flash mob in Manhattan in May 2003 and the first successful flash mob came together on June 3, 2003 - after the first try was foiled at Macy's department store．Lasik claimed that the activity was designed to make fun of hipsters （起时髦的人）, and call attention to the cultural atmosphere．

    Flash mob gatherings can sometimes shock people．Such an activity might seem amusing and untrue, but it also might frighten people who are not aware of what is taking place．Undoubtedly, flash mobs can serve as good political tools in any direction．They also have great economic potential, such as using flash mobs to advertise a product．

    The flash mob is now becoming more and more popular．People use it to do many things．For example, in 2009, Michael Jackson's fans took part in a flash mob to remember him．Hundreds of his fans gathered singing and dancing Michael's famous song "Beat It" together．Flash mobs give people from all walks of life an opportunity to come together to create a memory．

Metro Pocket Guide

Metrorail(地铁)

    Each passenger needs a farecard to enter and go out. Up to two children under ago five may travel free with a paying customer.

    Farecard machine are in every station, Bring small bills because there are no change machines in the station and farecard machine only provide up to $ 5 in change.

    Get one of unlimited Metrorail rides with a One Day Pass. Buy it from a farecard machine in Metro stations. Use it after 9:30 a. m. until closing on weekdays, and all day on weekends and holidays.

Hours of Service

Open: 5 a. m Mon-Fri           7a. m. Sat—Sun .

Close: midnight Sun—Thurs 3 a.m. Fri.—Sat. nights.

    Last train times vary. To avoid missing the last train, please check the last train times posted in the station.

Metrobus

    When paying with exact change, the fare is $ 1. 35 . when paying with a smarTrip card, the fare is $1. 25

Fares for the Senior /disabled customers

    Senior citizens 65 and older and disabled customers may ride for half the regular fare. On Metrorail and Metrobus, use a senior/disabled farecard or SmarTrip card. For more information about buying senior/disabled farecards, farecard or SmarTrip cards and passes, please visit MetroOpenDoors. com or call 202-637-7000 and 202-637-8000.

    Senior citizens and disabled customers can get free guide on how to use proper Metrobus and Metrorail services by calling 202-962-1100

Travel tips （提示）

. Avoid riding during weekday rush periods –before 9:30 a. m. and between 4 and 6 p. m.

. If you lose something on a bus or train or in a station, please call Lost & Found at 202-962-1195.

    Social media is our new travel agent, and it's changing the places we go. In some cases, the places we go. In some cases, the newfound fame brought about by social media platforms has helped to boost local economies and bring tourists to places they might never have discovered otherwise. In other cases, it's creating problems for countries and cities that are simply not equipped to deal with the influx(涌入) of tourists. Check out two of the incredible locations around the world that have gone viral on social media.

    Iceland

    This northern island nation has seen a dramatic surge(激增) in visitors over the past few years. One reason for that is the so-called "Game of Thrones effect,". This, combined with the availability of cheap flights and some very effective marketing campaigns, has made Iceland a popular travel destination. The number of tourists almost doubled from 566,000 to over 1 million between 2011 and 2015, according to Iceland's tourism board.

    While this influx of visitors has brought a much-needed boost to the economy post-recession(经济衰退), it's also pushed up prices for residents and put pressure on the infrastructure. Locals complain of tourists destroying the fragile ecosystem and leaving a mess behind them as they trample over the formerly untouched natural sites.

    Cuba

    According to Cuba's ministry of tourism, four million visitors went to Cuba in 2016, an increase of 13 percent from the year before. The tourism boom has had some consequences for the country's residents, The New York Times reported in December 2016.

    The surge in visitors has led to a food shortage, and basic food items have become completely unaffordable for locals. Local hotels and restaurants are buying up supplies in bulk for guests, pushing up prices and leaving limited amounts for locals.

    The situation has been acknowledged by the Cuban government, who put caps on prices to make them more affordable for residents. This has only encouraged sellers to put products on the black market, according to The New York Times.

    Whether you prefer burning the midnight oil or going to bed early so you can get up at the break of dawn depends on your genes, according to experts.

    I jump out of bed each morning, eager to start an active day. But I can hear my neighbor's alarm clock ringing non-stop every morning and I doubt he gets to work on time.

    A lot of noise comes from his flat in the evening. He's happy to stay up watching TV till after midnight, while I go to bed early and try to sleep.

    Well, it might not be his fault after all. I'm called“a lark (百灵鸟)”and my neighbor“an owl (猫头鹰)”

    We all have inside “clocks”in the brain to control all kinds of bodily functions and it is reset every day by light. These inside clocks run to a different schedule in “larks” and “owls”. If you have a fast clock, you like to do things early, and if you have a slow clock, you like to do things late.

    Because we live in a 24/7 world, scientists believe it's important to understand a person's “chronotype?—the time of the day when they function the best. It could help us lead a healthier life.

    A US professor has studied sleeping patterns and thinks work times should be changed and made more individual to fit in with our chronotypes.

    And he has advice for those who can't choose their working hours:“If that's not possible, we should be more careful about light exposure,” says the professor. “You should try to go to work not in a covered vehicle but on a bike. The minute the sun sets we should use things that have no blue light, like computer, screens and other electronic devices.”

    A food chain is a simple way of explaining how each living thing gets its food. For example, a simple African food chain might consist of three parts: first, trees and bushes; second, giraffes; and third, lions. Each link in a food chain is food for the next link. Food chains always start with plants and end with animals.

    Plants are at the bottom of the food chain. Scientists call them producers, because they use light energy from the sun to produce food from carbon dioxide and water through photosynthesis（光合作用）. Animals, unlike plants, can't produce their own food. Instead, they must eat plants or other animals. This is why scientists call them consumers.

    Consumer animals fall into three categories. Herbivores（食草动物）eat only plants. Carnivores（食肉动物） eat only other animals. Omnivores eat both plants and animals. In addition to producers and consumers, there are also decomposers(分解者).These organisms, such as bacteria and fungi, feed on decaying(腐烂) matter. They help the food chain by speeding up the decaying process that releases minerals back into the soil to be absorbed by plants as nutrients.

    Most food chains have only four or five links in them. As you go up a food chain, the amount of energy at each level diminishes, because some of the energy is lost in the form of waste or is used up by the organism at the level. That is why it takes many plants, for example, to feed a few giraffes who in turn feed one lion.

    Most animals are part of many different food chains, because they must eat more than one type of food to satisfy their energy needs. All of these interconnected food chains form a more complex structure called a food web. Humans, for example, are at the center of a very complex food web, because we tend to eat many different types of plants and animals.

    The largest genetic study of mosquitoes has found their ability to resist insecticides(杀虫剂) is evolving rapidly and spreading across Africa, putting millions of people at higher risk of contracting malaria(疟疾).

British scientists who led the work said mosquitoes' growing resistance to control tools such as insecticide-treated bed nets and insecticide spraying, which have helped cut malaria cases since 2000, now threatens "to disturb malaria control" in Africa.

    "Our study highlights the severe challenges facing public efforts to control mosquitoes and to manage and limit insecticide resistance," said Martin Donnelly of the Liverpool School of Tropical Medicine, who worked on the study with a team from Britain's Wellcome Trust Sanger Institute.

    Latest World Health Organization (WHO) data show that 216 million people were infected last year with the malaria parasite(寄生虫), which is transmitted by blood-sucking Anopheles mosquitoes. The disease killed 445,000 people in 2016, and the majority of them were children in sub-Saharan Africa.

    To understand how mosquitoes are evolving, the researchers sequenced the DNA of 765 wild Anopheles mosquitoes taken from 15 locations across eight African countries. Their work, published in the journal Nature on Wednesday, created the largest data resource on natural genetic variation for any species of insect.

    Analyzing the data, the scientists found that the Anopheles gamblae mosquitoes(冈比亚疟蚊)were extremely genetically diverse compared with most other animal species. This high genetic diversity enables rapid evolution, they said, and helps to explain how mosquitoes develop insecticide resistance so quickly.

    The data also showed the rapid evolution of insecticide resistance appeared to be due to many previously unknown genetic variants(变体)within certain genes. The scientists said these genetic variants for insecticide resistance were not only emerging independently in different parts of Africa, but were also being spread across the continent by mosquito migration.

    Michael Chew, an expert at Britain's Wellcome Trust global health charity which helped fund the research, said the finds underlined the importance of pushing scientific research ahead to control malaria.

Global efforts to control malaria through effective vaccine, insecticides and the best drug combinations require urgent, united action by scientists, drug companies, governments and the WHO.