Have you ever heard someone use the phrase"once in a blue moon"？People use this expression to describe something that they do not do very often．For example，someone might say that he tries to avoid eating sweets because they are unhealthy，but will eat chocolate"once in a blue moon"．Or someone who does not usually like to go to the beach might say"I visit the shore once in a blue moon．"While many people use this phrase，not everyone knows the meaning behind it．

    The first thing to know is that the moon itself is never really blue．This is just an expression．In fact，the phrase"blue moon"has to do with the shape of the moon，not the color．

    As the moon travels around the earth，it appears to change shape．We connect names with certain shapes of the moon．For example，when we can see a small part of the moon，it is called a crescent moon．A crescent is a shape that looks like the tip of a fingernail（指甲尖）．When we cannot see the moon at all，it is called a new moon．When we can see the whole moon，it is called a full moon．Usually，there is only one full moon every month．Sometimes，however，there are two full moons in one month．When this happens，the second full moon is called"a blue moon"．

    Over the next 20years，there will only be 15blue moons．So you can seldom see a blue moon． This fact has led people to use the expression"once in a blue moon"to describe things they don't often do in their lives．

    Is human blood all the same? "Sure," you may say. "It's all red." Well, it's not as simple as you might know. Blood is made up of different pans, such as red blood cells, white blood cells, and platelets (血小板). Two chemical molecules (分子) on the red blood cells make people's blood different. Scientists mark one molecule "A" and the other molecule "B".

    According to which and how many molecules are on the red blood cell, scientists divide blood into four types: A, B, AB and O. Your blood type will stay the same through your whole life. Scientists have been wondering if there are some links between people's blood type and their personalities (个性), but they don't know for sure. Some say that people with the same blood type may have things in common.

    What's your blood type? Does your friend have the same blood type as you? Read the following.

    People with type A: They look peaceful from the outside. But they ask so much of themselves that they may be very agitated inside. They become happy or sad easily. Sometimes they are shy but always very careful. They can easily win people's trust.

    People with type B: They have strong minds. Once they begin a task and make up their minds to do it, they will not give up easily. They'll try their best to finish it and do it well. They always try to make themselves different from others. They want to do things in their own way.

    People with type AB: Maybe they are the strangest among all blood types. They can be both open and shy, noisy and quiet at the same time! They are responsible and always ready to help others.

    People with type O: They're open and good at making friends. They always begin tasks but don't always finish them in the end. They have lots of new ideas and love to be the centre of attention. They always strongly believe in themselves.

    You know how much your telephone has changed over the past 10 years? Your car will change even more than that in the next 10 years. One of the biggest changes is that cars will drive themselves.

    "We certainly have the technology for it now." says Andrew Poliak of automotive(机动车辆的)technology supplier QNX." We expect self-driving cars to be a mainstream thing between 2020 and 2025."

    The American company Google has been working on self-driving cars for years. These cars are already on the roads in the United States and are truly self-driving. They have to steering wheels(方向盘）or pedals(踏板). Last week, the police ordered one of Google's cars to stop for driving too slowly on a public road. The car was not breaking any law, so no one was punished.

    Another American company, Tesla, added an "Autopilot" feature to its cars last month. With the Autopilot turned on, the vehicle drives itself. The car will speed up, slow down, brake and drive by itself. You can take over driving any time by turning the steering wheel or touching a pedal. The car uses sensors to know when driveway changing is safe and whether it should speed up or slow down.

    Tesla Autopilot is made for cross-country driving. It keeps you in the driveway and helps you avoid hitting other cars. It does not work well on local roads, for it will not stop at a red light or stop sign.

    Reporter Carolyn Nicander Mohr tried the Autopilot feature of a Tesla earlier this month. She had a hard time trusting the car to do what it should do. She thought about disabling the Autopilot feature many times during her trip. She wanted to take control at every bend in the road and hit the brake when the car in front of her slowed down. Yet the car drove perfectly.

    Other companies are working on self-driving cars, too. Marcedes-Benz, Audi, BMW. Volvo and Toyota all have plans for such vehicles.

Mother's Day is coming. It's a time to let mom know she's special to you. {#blank#}1{#/blank#}Finding the right gift for mom on that day is not always easy.We often want something personal and meaningful.But how can we make it？These following ideas may help you.

Gift of Time

Mom just want to spend time with their children to make it a Mother—Daughter/Son Day.花一整天时间和她在一起。{#blank#}2{#/blank#}Do the things she likes doing with her.

Home Video

Take your family videos and make them a short home video.She can enjoy it whenever she misses you.即使它可能会花费你大量时间，可是这个过程会很有趣。{#blank#}3{#/blank#}

Write a Letter

Write a letter instead of a simple card.Write about all the good memories you have with her and all the things she has done for you.

Flowers

People send carnations （康乃馨）on Mother's Day.You can easily spend a lot of money on them. If you have flowers in your garden，or can pick them by hand，it can be a great gift. {#blank#}4{#/blank#}

Dinner

Make a dinner and invite her over.You can also take it over to her house and surprise her. Just don't forget to wash the dishes after the dinner. {#blank#}5{#/blank#}

Many people think of cities and nature as two different things. However, some cities around the world are trying new and creative ways to bring nature and wildlife into city areas. 

In order to make better use of city gardens, places like Los Angeles in the US make community gardens. The gardens serve the community in many ways. In city farms, people grow vegetables to sell at markets with the money going to people in need. In the educational gardens, people are taught how to eat healthily. 

In Edmonton, Canada, more than twelve wildlife bridges have been built. One of the largest is a 30-meter-long bridge near Big Lake in the northwest of the city. It's a good way to protect animals' habitats（栖息地）. 

A "sponge city"（海绵城市）is a city to deal with floods. How does it work? Just as a sponge can absorb（吸收）water, a sponge city can deal with heavy rain. Wuhan is one of China's first cities to start the project. By the end of 2020, 20% area of the city has been covered by sponges. In 2030, the number will rise to 80%. There are now more than 30 "sponge cities" in China, including Jinan and Xiamen.