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 cannot make the water very hot. Although the air over the earth becomes hot, the air over the water is still 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.

    Then you may ask, "Why is the ocean full of salt?" Scientists will say that the salt comes from the rocks. When the rocks get very hot or very cold, they crack (爆裂). Rain falls into the cracks. The rain then carries the salt into the earth and into the rivers. The rivers carry the salt into the ocean. And then you may doubt, "But the ocean does not get more salty every year. Scientists are not sure about the answer to this question.

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

    What do you usually do in your spare time? Chatting online is now a national pastime in China, especially among the post-90s and post-00s generation. To some people, however, online chats can seem like a kind of polite conversation. Yet hidden rules and standards do exist, as long as you take the time to look for them.

    Get to the Point

    When sending a friend request, include your real name and any other identifying information. Remember, everybody's time is precious. If there's something you want, cut right to the chase. Never send an "Are you there?" without any follow-up messages. The quicker that you get to the point, the more likely the other person is to reply.

    Send the Right Things

    Please send texts, not voice messages. It might be easier for you to speak than type, but what about the person on the other end? They may be unable to listen to a 59-second message right now. Also, never send personal information by voice message—it is hard to note down. Emojis, GIFs and memes can save the day when you find yourself in an embarrassing conversation, or words fail you.

    Try to Understand

    You can always judge a person's mood by their text messages. If the person you're talking to says "Good night" or "I'm going to take a shower." that's probably not what they're actually going to do, it is just an excuse to stop talking to you. If someone hesitates for longer than three seconds before saying "Yes", they probably really mean "No". If you receive these emojis from friends who are older, read them literally (照字面地) (as a smile, a wave or a laugh). For the post-90s and post-00s generation, however, they could be speechlessness.

    Vienna is a beautiful old city on the River Danube in the center of Europe. It's the capital city of Austria and the center of European classical music. In the 18th century a lot of musicians came to study and work in Vienna.

    In the Strauss family, there were two composers called Johann Strauss: the father and the son. The father, Johann Strauss the elder, wrote and played music for traditional dances, called the waltz. His dance music made him famous all over Europe. The son, Johann Strauss the younger, was also very successful and popular. He wrote over 150 waltzes. In 1867 he wrote The Blue Danube waltz.

    Mozart was another very important composer. He was born in Austria in 1756. Before he was six, he played not only the piano but also the violin. His family took him around Europe and he gave concerts in many cities. He wrote hundreds of wonderful pieces of music. But he became very poor and died in 1791 when he was only 35. Like Johann Strauss, father and son, he was a great European musician, and many people still think his music is perfect.

    Imagine you are taking a walk on the beautiful beach. Just then, your feet step over something and you hear a crack (噼啪声) —an empty plastic bottle. Things like this are happening more often because of sea rubbish.

    Studies show that 80%of all the waste is from the land. The waste is mainly plastics (塑料制品) which are really hard to degrade (降解). So the rubbish will remain there for a long time. Some of the pollution is caused accidentally. For example, about 10,000 product-filled boxes are lost at sea every year from business ships. Sometimes people just throw things into the water from boats or land. Many large rubbish areas are formed in the North Pacific Ocean.

    Thankfully, the situation isn't ignored (忽视). Groups like the National Oceanic and Atmospheric Association (NOAA) are taking action to help clean the sea. Since 2006, 76 projects have cleared out 3,814 metric tons (公吨) of rubbish from the sea. Methods of rubbish cleaning include using machines to remove (移除) the waste away from the surface of sea water. Besides, special nets are set by the mouths of rivers to stop the rubbish before it reaches the sea. If everyone does his part, sea rubbish seems like a problem that can be solved.

    Laughter is natural for people. We start to laugh at about four months of age. We start to laugh even 1 we start to speak.

    Laughter connects us with others. We laugh more when we are with other people. It's difficult to pretend to laugh. Laughter is 2. When people pretend to laugh, most people know it's not real.

    When do people laugh?

    Only 10 to 20 percent of laughter is about something 3. Most laughter is about being friendly with other people. Most laughter says, "I don't want to 4 with you. I want to be friendly with you." This kind of laughter brings people together.

    We often laugh when we feel nervous. In movies, there is often a joke at an exciting moment when everyone feels nervous. It is usually a(n) 5 joke, but we laugh a lot. Our laughter helps us 6.

    Why doesn't everyone laugh at the same joke?

    Not everyone has the same sense of 7. Some people think a joke is funny, but other people don't think so. People have different ideas about what is funny.

    Our idea of what is funny changes with 8. For young children, the world is new. They are interested in many things, so they laugh 9. Teenagers often laugh to protect themselves. Adults laugh at themselves and other people with similar problems. They laugh at things that give them stress. Our 10 for laughter change gradually.

    Grandparents are respected in many human societies. But telling stories about old times and overfeeding grandchildren seem like human qualities. Are these classic grandparent behaviors really limited to humans?

Do any animals know their grandparents the way people do?

For most species on Earth, the answer is certainly " No" . " Usually, there aren't grandparents around anymore when an animal is born,"  said Mirkka Lahdenpera, a biologist at the University of Turku in Finland. " Even if an animal's life span does overlap (重叠) with its grandparents', most species spread out to avoid competing for resources, so the chance of running into a grandparent is slim." 

But there are a few clear exceptions, primarily among mammals (哺乳动物) that live in close social groups. In troops of langur monkeys in India, older females lived together with their daughters and grandchildren. The grandmother langurs defend the group's babies against attacks and even give their own grandchildren special treatment.

Many whale species, too, travel in family groups that include both grandmothers and grandchildren.

Elephant herds are also famously matriarchal. They are led by a grandmother, who can live to around 80 years old, and are generally made up of her daughters and their young. The females in a herd form close bonds and work together to raise their young.

Lahdenper found that the babies of young mothers were eight times more likely to survive if their grandmothers lived near them than if they didn't. When the young mothers were older and more experienced at raising babies, this beneficial grandmother effect disappeared even if the actual grandmothers were still around.

Indeed, most evidence for the benefits of grand parenting comes from mammals. But in 2010, researchers found that in troops of insects called gall-forming aphids, older females defend their relatives after they've stopped reproducing (繁殖).

And what about grandfathers? Studies of humans in recent decades have shown that a living grandfather can improve a person's mental health and other indicators of well-being. But there's no evidence of that in the animal kingdom. Male animals rarely socialize with their own children, let alone any grandchildren.