When the weather is hot, you go to a lake or an ocean. When you are near a lake or an ocean, you feel cool. Why? The sun makes the earth hot, but it can not make the water hot. Though the air over the earth becomes hot, the air over the water stays cool. The hot air over the earth goes up, then the cool air over the water moves in and takes the place of the hot air. When you are near a lake or an ocean, you feel the cool air when it moves in. You feel the wind and the wind makes you cool.

Of course, scientists cannot answer all of our questions. If we ask "Why is the ocean full of salt?" scientists will say that the salt comes from rocks. When a rock gets very hot or very cool, it cracks (崩裂) , and rain falls into the cracks. The rain then carries the salt into the earth and the rivers. The rivers carry the salt into the ocean. But then we ask "What happens to the salt in the ocean? The ocean does not get saltier every year." scientists are not sure about the answer to the question.

    We know a lot about our world. But there are still many answers that we do not have, and we are curious(好奇的).

    Stop reading for a minute and think about what you see when you look up at the sky on a clear night. You probably see many stars (星星). Long long ago, farmers watch some stars to know when to plant their crops. Travelers did so to find their way in the dark. Today stars are still important to some people.

    Scientists find that most stars are very big. In fact some stars are so far away that it would take millions of years to get to them even by spaceship.

    Another thing we know today about stars is that some are made up of(由…组成) hot gases. The gases are so hot that they send out light. If you watch a star carefully, you may notice that it twinkles(闪烁)sometimes. As you may understand, light from a star must shine through the air before you can see the star. Air is always moving, it makes light from the star move back and forth. When the light moves, it makes the star twinkle. Stars also have different colors. The colors of a star depend on how hot it is. To our surprise, the cool stars are red and the hottest stars are white.

    Butterflies are common all over the world, but butterflies in hotter countries are usually bigger and more brightly colored than those in cooler countries, and they are more different kinds.

    Usually the male(雄性的)butterflies are more brightly colored than the females. The females have bigger and heavier bodies since they carry the eggs. The bright colors of the males help to attract the females, and the duller(较暗淡的)colors of the females make them less likely(可能的)to be caught by enemies before they have laid their eggs.

    Some butterflies hare bright pattern colors on the upper surface of their wings that show when flying, but when they rest on a tree or leaves, they close their wings. On the undersides of their wings there are green and brown marks which look like the tree or leaves. They are then difficult to be seen. The best example of this is the Dead Leaf Butterfly. When its wings are closed, it looks exactly like a dead leaf. There are even marks that look like the holes in a dead leaf. When the butterfly is run after by a bird, it closes its wings and seems to have disappeared completely.

    Many butterflies have round marks on their wings that look like eyes. The round marks have a useful function(功能). When the butterflies are resting, their wings are closed, and these marks can't be seen. When a bird sees one, and begins to attack it, the butterfly will open its wings. The bird thinks that it sees two eyes staring at it, frightened, and flies away.

Doing morning exercises is a daily activity in every Chinese school. For the adults, however, familiar schoolyards have already {#blank#}1{#/blank#} (take) place of by public parks. 

The morning scenes in central parks around China are {#blank#}2{#/blank#} (live). At every corner of the park, you can also see people practising many kinds{#blank#}3{#/blank#} sports: square dancing, badminton and so on. 

But the most eye-catching groups in the park are the so-called "Kung Fu Grandpas/ Grannies". Most of them are common workers from different industries who didn't start morning exercises {#blank#}4{#/blank#} they retired. They say age is just a number as long as they expect {#blank#}5{#/blank#} (be) healthy. 

The {#blank#}6{#/blank#} (advantage) of exercise are endless: a healthier body together with a slimmer figure and a {#blank#}7{#/blank#} (happy) mood. No matter how old they are or when during the day they exercise, they will always strengthen the body. 

Social connection is also {#blank#}8{#/blank#}the olds search for. Feng, who is seventy-two years old, plays Taichi every day. He {#blank#}9{#/blank#} (keep) this exercise for six years and now he is managing a WeChat group of 270 members who share the same hobby. 

"What I like about this place is that newcomers are always welcomed. Everyone in the park feels {#blank#}10{#/blank#} (include)," said Feng. It is like a special party of later life. 

Humans are no stranger to taking on technological challenges. Hollywood movies have imagined a future of hoverboards(悬浮滑板) and flying cars. {#blank#}1{#/blank#} What's new and popular today? Let's take a look.

The Robotaxi

In a Robotaxi, no one drives, it drives by itself. You just tell where and it goes. It's like a magic ride, but a car! It watches the road with computers and cameras. {#blank#}2{#/blank#} They can reduce the number of cars on city streets by 60% and road accidents by 90%.

The Flying Cars

Flying cars include passenger drones(无人机) and helicopter hybrids(混动). {#blank#}3{#/blank#} Because the sky will never be as busy as the roads are. It may take you 30 minutes instead of 2 hours to go to a friend's house. Besides, it only costs around 60 dollars, and you get to fly. Isn't it cool?

The High-speed Trains

China is leading the world in high-speed trains. The latest type of trains can run at the top speed of 450 kilometers per hour. It has set a world record for the fastest train speed. The high-speed trains make the travel safer, greener and more comfortable. {#blank#}4{#/blank#} In recent years, these trains have been sold to 23 countries.

The Starship Rockets

SpaceX company's new rocket, Starship has completed its first flight. SpaceX engineers take this task as a great success. After a rocket finishes its job, it usually falls and breaks. But the Starship rockets don't break. By reusing the most expensive part of the rocket, it reduces the cost a lot. {#blank#}5{#/blank#}