Reading can be a social activity. Think of the people who belong to book groups. They choose books to read and then meet to discuss them. Now the website BookCrossing.com turns the page on the traditional idea of a book group.

    Members go on the site and register the books they own and would like to share. BookCrossing provides an identification number to stick inside the book. Then the person leaves it in a public place, hoping that the book will have an adventure, traveling far and wide with each new reader who finds it.

Bruce Pederson, the managing director of BookCrossing, says, “The two things that change your life are the people you meet and books you read. BookCrossing combines both.”

    Members leave books on park benches and buses, in train stations and coffee shops. Whoever finds their book will go to the site and record where they found it.

    People who find a book can also leave a journal entry describing what they thought of it. E-mails are then sent to the BookCrossers to keep them updated about where their books have been found. Bruce Peterson says the idea is for people not to be selfish by keeping a book to gather dust on a shelf at home.

BookCrossing is part of a trend among people who want to get back to the “real” and not the virtual(虚拟). The site now has more than one million members in more than one hundred thirty-five countries.

    Last year DeepMind's AlphaGo programme took on and beat two of the world's best players of the Chinese game of Go（围棋）-an unbelievable achievement seen as a milestone in the development of artificial intelligence (A). That programme was trained by first giving it vast amounts of data from amateur and professional games.

    The new AlphaGo Zero began with a blank board and nothing but the rules of Go and set about playing against itself. Within three days it was so advanced that it took on the previous version, which had taken months to develop, and defeated it by a hundred games to zero.

    DeepMind says that creating knowledge from first principles without learning from human expertise（专门技能）is an important step in developing artificial intelligence. The company's founder Demis Hassabis said this breakthrough could help in real world problems such as designing new drugs or discovering new materials.

    In 1997, Garry Kasparov was defeated by Deep Blue, a computer program written by IBM, running on a supercomputer. This was the first time that a ruling world chess champion was defeated by a computer program in tournament conditions. Superficially, AlphaGo's win against Lee Sedol can be compared to Deep Blue's win against Gary Kasparov, which happened almost 20 years ago. So: what's the big deal? We have to understand the differences between chess and Go.

    In chess, each player begins with 16 pieces of six different types. Each piece of type moves differently. The goal of the game is to seize the other player's king. Go starts with an empty board. The goal of the game is to gain as much territory（领地）as possible.

    Although the rules of Go might appear simpler than those of chess, the complexity of Go is higher. Also, games usually last longer for lots of moves. A typical game in Go might last for 150 moves vs.80 in chess.

    If you're new to San Francisco, paying the city a visit, or simply looking for a new playground for you and your dog, here are four of the finest dog parks in San Francisco.

    Corona Heights Dog Park

    Often visited by residents from all over the city because of its nearness to the Randall Museum, Corona Heights Dog Park offers pets and owners breathtaking view after a steep hillside climb, and a fenced dog area that allows dogs to let loose without a leash（狗链）. Owners will also enjoy the playground, tennis courts and basketball courts.

    Dolores Dog Park

    The grounds of Dolores Park once served as camps for those who were left homeless by the 1906 earthquake, but now are often visited by crowds of Mission District people. Four-legged friends can also wander about carefree off-leash while making friends with other members of the doggie community.

    Pine Lake Dog Park

    Famous for its place as a rest stop for hundreds of species of birds to fly to and from warmer climates, Pine Lake Park is also prized by city dogs and their owners for their nice hiking paths, picturesque lake suited for swimming, and off-leash area on the park's west end that lets dogs run free.

    Buena Vista Dog Park

    With a history of 146 years, Buena Vista Park is San Francisco's oldest park. Dogs and owners with plus energy will love burning it on this park's steep paths and winding staircases. Dog owners should have good control over their dogs, as it's quite easy for dogs to get separated when going through Buena Vista's many twists and turns.

    The year was 1932. Amelia Earhart was flying alone from North America to England in a small single-engined aeroplane. At midnight, several hours after she had left Newfoundland, she ran into bad weather. To make things worse, her altimeter (高度表) failed and she didn't know how high she was flying. At night, and in a storm, a pilot is in great difficulty without an altimeter. At times, her plane nearly plunged (冲) into the sea.

    Just before dawn, there was further trouble. Amelia noticed flames coming from the engine. Would she be able to reach land? There was nothing to do except to keep going and to hope.

    In the end, Amelia Earhart did reach Ireland, and for the courage she had shown, she was warmly welcomed in England and Europe. When she returned to the United States, she was honored by President Hoover at a special dinner in the White House. From that time on, Amelia Earhart was famous.

    What was so important about her flight? Amelia Earhart was the first woman to fly the Atlantic Ocean alone, and she had set a record of fourteen hours and fifty—six minutes.

    In the years that followed, Amelia Earhart made several flights across the United States, and on each occasion (时刻) she set a new record for flying time. Amelia Earhart made these flights to show that women had a place in aviation (航空) and that air travel was useful.

    As scientists in the Netherlands tried to figure out how to build a super flying robot, they learned from one of nature s flyers: the humble fruit fly(果蝇).And by building this robot, they've gained new insights into how the fly carries out one of its dangerous tasks.

    The robot is called the DelFly Nimble Its wingspan(冀展)is about a foot wide. It has four wings that can beat at 17 times per second, which appear very delicate because they're made of the same material as space blankets.

    In the previous designs, they always had a tail, like a traditional airplane tail, said the robots main designer Matej Karasek. He's based at the Micro Air Vehicle Laboratory at Delft University of Technology in the Netherlands, and he and his colleagues published their findings on Thursday in Science

    “In the previous generations", he said, "flapping wings drove the robot forward while the tail helped to guide and stabilize it. But now the DelFly Nimble is completely controlled by the wings. The challenge then was actually combine the control into the wing movement, and that's what we achieved, Karasek said

    In the latest generation. the wings can each move individually or rotate(旋转)around the body of the robot in order to maximize the machine s agility(灵活 ) The robot can remain in one place in the air for about five minutes on a full battery or fly for more than a kilometer"， Karasek said, "And because the scientists are controlling all the movements, they can use the robot to learn more about how fruit flies actually carry out their dangerous tasks, which has caught the attention of biologists.”

    As winter set in, it's the perfect time to get out of town and escape the seasonal mood disorder. Here are vacation ideas to adopt now.

    Costa Rica

    Costa Rica, located in the Valle del General, is an off-the-radar agriculture town that's not overrun with tourists. The area is known for its coffee, pineapples and sugar cane plantations, as well as its rich forests. What's more, you can enjoy horseback riding, spend afternoons at the spa or tour a nearby wildlife shelter to look out for colorful birds, sloths and other animals.

    Los Cabos, Mexico

    If you love wine, travel to Los Cabos, a beautiful vacation destination at the southern edge of the Baja Peninsula. During your getaway, you can enjoy special wine tastings and dinner cooked by Los Cabos's Michelin-starred chef. There are wildlife-viewing opportunities, too. From December through May, you can spot whales, manta rays and more wildlife.

    Petit St Vincent, the Grenadines

    This private Caribbean island was spared the anger of hurricanes Irma and Maria. What's more, the whole island is only 115 acres and has 22 cottages. Each cottage is located in a quiet place, and you can expect to unplug and recharge phones with no wireless connections, televisions or telephones in the cottages. Even better, you can spend your days admiring fascinating marine (海洋的) life at the Jean-Michel Cousteau Diving Caribbean center.

    The Exumas, Bahamas

    On Fowl Cay's 50-acre private island, you can enjoy a carefree escape. Days can be spent fishing, boating and diving before wandering along white sands. The resort features large one-, two- and three-bedroom houses. Best of all, if you're planning a group getaway or multigenerational trip, you rent the entire island for a one-of-a-kind island escape.