Car companies are developing vehicles that will plug into electric sockets, ust like many laptops, digital cameras, cell phones and Mp4 do. Called “plug-in vehicles”, these cars will get most of their power from electricity. Their drivers won't have to stop at gas stations as often as usual.

    The technology is more than just cool. In our car-filled world, plug-in vehicles could reduce the amount of gas we use, which keeps rising in cost now and then. Besides, driving around in these vehicles may even help the environment. Gas-burning cars produce a lot of greenhouse gas, which causes global warming.

    The first company-produced plug-in vehicles could hit the roads by 2020. But engineers still have a lot of work to do to make the technology practical and inexpensive.

    Batteries are the biggest challenge. In the plug-in-vehicle world，Li-ion(锂离子)batteries are getting the most attention. These batteries can store a large amount of energy in a small package, and they last a longer time between charges. Li-ion batteries can fit laptops, cell phones, heart instruments and other similar pocket ones.

    But because cars are so big and heavy, it would still require a suitcase-sized Li-ion batteries to power about 12km of driving. What's more, the batteries are much expensive.

    “A car filled with batteries could go a long distance,” says Ted Bohn, an electrical engineer in Chicago. “But it couldn't pull any people, and it would cost $100,000.” So researchers need to work out how to make batteries smaller and cheaper, among other questions.

    “The answers don't exist yet,” Bohn says, “As a kid, I thought someone someplace knows the answer to everything. All of these questions haven't been decided. That's what engineering is about-making a guess, running tests and getting fine results.”

    Mountaineers have noted that as they climb, for example, up to the 12,633­foot Humphreys Peak in Arizona, plant life changes greatly. In the Sonoran Desert, one climbs into a pine forest at 7,000 feet and a treeless tundra(冻土带) on the top of the mountain. It may seem that plants at a given altitude are associated in what can be called “communities” －groupings of species. The idea is that over time, plants that require particular climate and soil conditions come to live in the same places, and are frequently to be found together. Scientists study the history of plant life and build up a picture of how groups of plants have responded to climate changes and how ecosystems develop. But are these associations, which are real in the present, permanent?

    A great natural experiment took place on this planet between 25,000 and 10,000 years ago, when small changes in the earth's orbit caused great sheets of ice to spread from poles. These glaciers(冰川) covered much of North America and Europe to depths of up to two miles, and then, as the climate warmed, they retreated. During this retreat, they left behind newly uncovered land for living things to occupy, and as those living things moved in they laid down a record we can read now. As the ice retreated and plants started to grow near a lake, they release pollen(花粉). Some would fall into the lake, sink to the bottom and mix with the sand. By drilling into the lake bottom it is possible to read the record of the plant life around the lake. The fossil record seems clear; there is little or no evidence that entire groups of plants moved north together. Things that lived together in the past don't live together now, and things that live together now didn't live together in the past. Each individual living things moved at its own pace. The fossil record seems to be telling us that we should be thinking about preserving species by giving them room to move about－to respond to environmental changes.

    Everglades National Park (大沼泽地国家公园) is located in the state of Florida. It is the largest wilderness in the entire country and makes up 25% of the wetlands in the state. The park is home to several rare and endangered species. It is also the third largest national park in the US, after Death Valley and Yellowstone. Each year, about 1 million tourists visit the park. On a global level, it has been announced as a World Heritage Site.

    Unlike most other national parks, Everglades National Park was created to protect an ecosystem (生态系统) from damage. In 1947, President Harry Truman spoke at the official opening of Everglades National Park, saying the goal of creating the park was to protect forever a wild area that could never be replaced.

    10,000 different islands make up Everglades National Park. Each of these islands is lived by natural wildlife. The Everglades is home to about 15 species that are endangered. In addition, more than 350 bird species and 300 species of fresh and saltwater fish live within the park. The Everglades is also home to 40 species of mammals and 50 reptile species.

    There are many ways to explore the Everglades. Visitors can see alligators (短吻鳄) while hiking the Anhinga Trail. The Everglades is one of the only places on Earth where freshwater alligators and saltwater crocodiles live in the same area. Visitors using airboats are likely to see large groups of birds. Some visitors might enjoy riding bicycles through Shark Valley. Others may want to move slowly through waters where they can see insects and wildlife closely.

    According to experts, changes to the Everglades are becoming a danger to several different kinds of wildlife. They say it is a result of actions the US government began more than 50 years ago, and settlers began even earlier.

    Audrey Hepburn(奥黛丽·赫本) won an Academy Award as Best Actress for her first major American movie, Roman Holiday, which was released in 1953. But she is remembered as much for her aid work as for her acting.

    Born in Belgium in 1929, Audrey's father was British and her mother was Dutch. Audrey was sent to live at a British school for part of her childhood. During World War II, she lived and studied in the Netherlands. Her mother thought it would be safe from German attacks. Audrey studied dance as a teenager and during college. But when she returned to London after the war she realized she wasn't going to be a ballerina (芭蕾舞女演员). So she began taking acting parts in stage shows. Late she began to get small parts in movies.

    But it was Audrey Hepburn's move to America in 1950s that brought her true fame. In 1951 she played the character “Gigi” in the Broadway play of the same name and received great critical praise. Two years later, Roman Holiday made her a star at the age of 24.

    Audrey Hepburn made more than 25 movies. Among her roles, the most popular one was Holly Golightly in Breakfast at Tiffany's in 1961. Three years later she played Eliza Doolittle in My Fair Lady.

    She was married two times and had one child with each husband. In 1989, the UN Children's Fund named Audrey a goodwill amassador. She travelled all over the world in support of UNICEF（联合国儿童基金会）projects. The UN agency said she was a tireless worker. She often gave 15 interviews a day to gain money and support for UNICEF projects.

    Audrey Hepburn often said her loyalty to UNICEF was the result of her experiences as a child during World War II. She said she knew what it was like to be starving and to be saved by international aid. She was a goodwill amassador until her death in 1993 from colon cancer.

    In recent years more and more Chinese schoolchildren are required to wear uniforms at school. Do American children wear school uniforms? In fact, American schoolchildren often wear uniforms if they attend religious or other private schools. Most public schools do not require uniforms. But over the last ten years or so, more of them have moved in that direction, including high schools. Another option is that students may just have to dress alike—for example, white shirts and dark colored pants or skirts. Even schools that do not require uniforms generally have a dress rule about what they consider acceptable. Schools commonly forbid clothing that shows images or words causing displeasure, or simply too much skin. Hats may be forbidden because, for example, different colors may be connected with bad guys.

    Some parents like the idea of uniforms. Some say it means they do not have to spend much on clothing for their kids. Others, though, argue that uniforms represent an unnecessary cost.

    There are also debates about whether uniforms or other dress rules go against civil rights. Students and parents have taken legal action against school dress requirements. Several years ago, a middle school was asked to stop its dress rule unless families had a way out of it. The American Civil Liberties Union of Northern California brought the case for the families of several students who had been punished by the school. Most attention centered on a girl who wore socks with the Tigger character from Winnie the Pooh. The school said its clothing policy, including no pictures of any kind, was needed to control a growing problem with gangs. The families argued that the policy went against free speech rights as protected by the United States and California constitutions.

    The United States Supreme Court says student expression is protected as long as it does not harm the work and the rule of a school. But some educators believe dressing alike helps improve student learning. They believe that uniforms help create a sense of unity and reduce the risk of fights. They also say uniforms make it easier for security reasons to tell if someone belongs to the school or not.

VAZOnepwr Blade 5 Dual

Cordless Blade 5 is a great one for sheer suction(吸力) power. It's easy to use it under furniture and it works well on all surfaces. The dust tank holds 0.7 litres and it comes with two batteries for 90 minutes of charge. If you're on a budget, the Blade 4 is a good option (now£250,from£450;vax. co.uk)

MIele Complete C3

At 890W, this corded vacuum is "exceptionally powerful" and its "Air Clean" filtration traps even tiny particles. It has a 12m radius(半径) and comes with three tools, stored in the body(£219;miele.co.uk)

Shark CZ500UKT Duo Clean

This 880W corded vacuum has remarkable suction power. It can switch between cleaning carpets or hard floors, and has an anti-hair-wrap brush roll and an LED light to show up dust (£330; sharkclean.co.uk)

Dyson Ball Animal Multi-floor

Unusually easy to move or turn skillfully thanks to its responsive rollerball, this bagless vacuum delivers strong suction and is great for getting up pet hair. A bit heavy at 7.4kg, it has a ten-metre cord and 3.5-litr e caddy, which ejects at the touch of a button (£380; dyson.co.uk)

Henry HVR 160-11

A long-standing favourite, the Henry is famously strong and great value. It has a large six-litre dust bag, and packs 620W of power. Its cord is a generous ten metres, and it has a long 2.2m hose, which is useful for cleaning up high and under sofas (£130; myhenry.com).