What is the sky? How high is it? What color is it? What birds can live above it? These questions are difficult to answer, aren't they? In fact, the sky has no color. Then why does the sky look blue?

    We know that there is air around the world. When planes fly, they need air to lift their wings. Planes can't fly very high. That is because the air gets thinner when they go higher. The same will happen to us if we go high enough. If we go up about 1,200 kilometers from the earth, we'll find no air there. Some birds can fly into the sky as high as a plane, but they cannot fly or live without air.

    Looking up above us, you will see the blue sky with white clouds. It is usually light blue. The color would be bluer when you see on the top of a high mountain. And you will see fewer clouds in the sky between you and the sun. Sometimes the sky doesn't look blue. When the sun rises and falls, you will see red, orange and other colors in the sky, such are the most beautiful moments of the day.

    Who can see the colors of the sky high above the earth? The astronauts! They can see those beautiful colors of the sky from the outer space. However, before flying into the outer space, they sit in a spaceship high above the earth and there is no air at all. The sky would be that dark and its color would be black.

    Now you may understand that we see colors because the air scatters (使散射) the sunlight. Light travels through the air. Without air there would be no colors.

Millie打算向美国的学生介绍自己的国家，根据表格里的要点，写一篇文章介绍中国。

注意： 1. 不得出现自己的学校和真实姓名。2. 必须包含所提示的内容，可以适当发挥。3. 词数：90词左右。

Since May 8, Boston Dynamics' dog-like robot, Spot, has been in use at Bishan-Ang Mo Kio Park in Singapore to help encourage social distancing during the COVID-19 pandemic (新冠肺炎疫情) according to a report in the South China Morning Post.

Singapore's government is funding the experiment of the robotic dog, which will last for two weeks at the park during off-peak (非高峰的) hours.

"The robot isn't enforcing (强制). It's just giving people information and encouraging them," Boston Dynamics founder Marc Raibert told CNBC. "When Spot is patrolling (巡逻) the area, there's a park officer nearby who can do whatever enforcement he decides is suitable."

Cameras fixed on Spot will scan the surroundings and help officers work out the number of visitors in the park, but they cannot read faces and won't collect any personal information. It also carries a loudspeaker to broadcast a pre-recorded message to remind visitors of the importance of social distancing.

Without enough manpower, Singapore is turning to robotics to reduce the manpower required for park patrols and remind people about the risks they may be putting themselves and others in.

The question, however, is whether their use of Spot will be a good tool or will only attract more people to it instead.

If the two-week experiment proves successful, the government will consider using Spot at the park during peak hours. Singapore will also do studies to see if it's worth using other Spots in other parks. Already, Singapore engineers are trying to improve Spot, making it tell if people are together in a group or strangers passing on the grass.

No one knows if Spot will stay after the pandemic is gone, but one thing is for sure: the robot is the hardest working dog in the world.

There is a small thing we use every day. We usually put {#blank#}1{#/blank#}in the bathroom. Without it, we might have tooth problems. Can you guess {#blank#}2{#/blank#} it is? Yes, it's the toothbrush. But do you know anything about its history?

According to some research, before toothbrushes appeared, the ancient Chinese people {#blank#}3{#/blank#}their teeth with some small tree branches (树枝). About 800 years ago, people made a kind of brushes with pig hair and bamboo. It might {#blank#}4{#/blank#} the world's earliest form of toothbrushes. As time went{#blank#}5{#/blank#}, the forms of toothbrushes changed. For example, people chose horse-tail hair instead of pig hair {#blank#}6{#/blank#} pig hair was too hard. Imagine that you travel back to {#blank#}7{#/blank#}of years ago and brush your teeth comfortably. It sounds fantastic, doesn't it?

Around the 15th century, the Chinese toothbrush found its way into Europe. It became {#blank#}8{#/blank#} among local people soon. The {#blank#}9{#/blank#} of toothbrushes was not as famous as that of paper-making, but it was very useful in daily life. Today we have a good {#blank#}10{#/blank#} of brushing teeth. We should thank the ancient Chinese for it.

A..Have the willingness to change an opinion in the face of better data. 

B.Reading is a good way to stay open-minded.

C.A system can be measured,refined,and optimized.

D.The better we get at writing the better we get at thinking. 

E. Argue with yourself.

F.They only make a conclusion once they have studied a topic in depth.

There is no quick hack to becoming a critical thinker but there are some habits that one can build to after their character in the right direction.

● Read a lot and read widely

{#blank#}1{#/blank#} Well read people understand that there are different ways to look at a problem and different lenses with which to view the world.

● Don't jump to conclusions

A good critical thinker has built up the habit of not jumping to conclusions especially when there is no data.{#blank#}2{#/blank#}

● Create systems,not goals

For example,creating a system habit that promotes writing every day is more effective than setting the goal of writing an essay a week. {#blank#}3{#/blank#}.A goal only focuses on the outputs.

●{#blank#}4{#/blank#}

Great thinkers routinely and intelligently try to counter their own arguments.They go back and forth until only the most rational and strongest argument exists based on the knowledge they have.

● Write

Writing is thinking. {#blank#}5{#/blank#}.Writing also helps us separate what we actually know from what we think we know.

How to have good friends? Here are some tips for you.

Choosing Friends

Before you make friends, you have to decide who you want to be your friends. Most people like to have friends with the same interests. But that doesn't mean you have to be like each other. It's OK if you enjoy some of the same things.

Making Friends

●The quickest way to make friends is to smile. Remember that most people hate an angry face

●One easy way to star a conversation with someone is to say something nice. For example, you could talk about a great answer to a teacher's question,a good catch at the ball field and so on.

●Ask your new friends questions about themselves." Who's their favorite singer? Where do they live? What do they do after school?" are all good questions

●Be a friend. If you show an interest in other kids and are kind and friendly, you can make good friends.

Having Activities

It's always fun to play your favorite activities with you friends together. For example, inviting a friend to your house is a great way to make your friendship closer. Make sure the activities can be done together. You may both love playing computer games, but if one person is playing and the other has nothing to do but watch, it's not fun.

Friendship is an important part in our life, because friends make us happy and support us through hard times. If you are still waiting for your" friends", start with the tips above.