We do not know when man first began to use salt, but we do know that it has been used in many different ways throughout history. Historical evidence shows, for example, that people who lived over 3,000 years ago ate salted fish. Thousands of years ago in Egypt, salt was used to preserve (保存) the dead.

    Stealing salt was considered a major crime (罪行) during some periods of history. In the eighteenth century, for example, if a person was caught stealing salt, he could be put in prison and his ears could be cut off.

    In the Roman Empire, one of the most important roads was the one that carried salt from the salt mines to Rome. Guards were stationed(安置) along the route to protect against salt thieves, and they received their pay in salt, thus bringing the English word, salary. Any guard who fell asleep while on duty was thought to be “not worth his salt”, and as a result he would get a little less salt on his next payday. The expression, “not worth his salt”, is still used today in English.

    In the modern world salt has many uses beyond the dining table. It is used in making glass and airplane parts, in the growing of crops, and in the killing of weeds (杂草). It is also used to make water soft, to melt (融化) ice on roads and highways, to make soap, and to fix colors in cloth.

    Salt can be got in various ways besides being taken from mines underground. Salt water from the ocean, salt water lakes or small seas can be used to make salt. Yet, no matter where it comes from, salt will continue to play an important role in the lives of people everywhere.

    When Luke went to university he thought he would be on a new journey in life and getting his own place. In the UK, it's common to fly the nest at a fairly young age. Many choose a flat-share; others make plans to get on the property ladder.

    But the current economic situation forced Luke back to his mum's house at the age of 27. And he's not alone: a quarter of young adults in the UK now live with their parents. The Office for National Statistics said more than 3.3 million adults between the ages of 20 and 34 were living with their parents in 2013.

    Lack of jobs and the high cost of renting accommodation made Luke change his plans. He's upset. "There's something very difficult about being an adult living in an environment where you're still a child," he says. "It limits me socially; sometimes I feel it limits me professionally."

    Indeed, many young people have no choice but to stay at "the hotel of Mum and Dad".

    Krissy had to return home after a year away and now lives in rather terrible conditions, sharing the family's three-bedroom house with her sisters. She says they end up getting on each other's nerves when it's time to use the bathroom in the morning.

    Of course, living with your parents is not unusual in some countries. Economic conditions, culture, or family traditions mean many young people stay at home until they get married. Even then, it can be too expensive to rent or buy a house and the married couples continue to live at one of their parents' homes.

    But some parents seem to enjoy having their kids back at home. Janice's daughters are part of what's being called "the boomerang generation". She says, "I get to share their lives with them, and I've got to know them all as adults. We have the sort of conversations that good friends do."

    So for some it's a win-win situation — spending time with your families, and saving money.

    If you are invited to a wedding, baby shower or other celebrations, you will be expected to bring a gift. Unless you know the host very well, the gift should be modest in value, about $25 or so.

    For a wedding, the bride will have "registered (登记)" at one or two local department stores，indicating the items she prefers. You can buy the couple a gift that isn't listed , but most people buy something listed on the registry (记录簿). If you buy an item listed on the registry, be sure to tell the store that you are doing this, so that the couple doesn't receive duplicate (重复的) gifts. For a baby shower, bring a gift appropriate for a new born baby.

    If you wish to give a gift when you leave to return to your country, the best gift   is something unique to your country. It does not need to be especially valuable or rare. Possibilities include a book about your country, an inexpensive piece of handicraft( 手工艺品)or art, or something else that reflects your culture. If the children collect coins and stamps, they would be very pleased with a set of your country's coins or a selection of stamps from your country.

    If you owe a debt of deep gratitude to an American host family, a common way of repaying it is to take the family to a form of entertainment, such as a baseball, basketball, or a good restaurant.

    Gift giving is not as important in America as it is in other countries, so there is nothing wrong with not giving a gift. If you need help, talk to a salesperson at a department store. Tell them about the person who will be receiving the gift and the reason for the gift, and they will help you find something appropriate.

    Being stuck at home doesn't mean that learning stops. In fact, it's just the opposite. Here are some activities and resources to foster kids' imagination and build their curiosity at home.

    Let's Get Down to the Science

The science lab at school may be closed, but you can still put on goggles and a white lab coat and learn at home. This Test Tube Experiment Kit will make you feel like a genius in no time. Kids will get the chance to learn about basic chemistry, physics, and more by creating pop-resistant bubbles, bouncy balls, and other objects.

    To Infinity (无穷) and Beyond

Earth is a part of the Milky Way galaxy, but did you know there are around 100 million galaxies in the universe? There are so many fun and fascinating things to learn about space that the possibilities truly go to infinity and beyond. A great way start your child's educational space adventure is with this Real Life Sticker and Activity Book.

    Take a Walk on the Wild Side

    Biology class is in session! Here's your chance to get up close and personal with a great white shark without putting on a wetsuit. This 4D Great White Shark Anatomy Kit allows you to get an inside look at what makes these predators of the deep so powerful. And then take an even deeper dive into a shark's world with I Am Shark.

    Take advantage of this time at home and find new ways to keep exploring the world around you.

We cannot argue with reality. We cannot argue with science. Therefore, we simply cannot argue with the fact that there are no straight lines in the universe.

Let's start with science. The science of a straight line falls under the subject of physics. It might seem like a complicated topic, but the theory behind it is pretty simple. If you start rowing in a boat from one place and keep sailing, the concept of the curved（弯曲的） Earth will take you in a circle and you will end up where you started. The brain forms the concept of a straight line to simplify what you see in nature. Consider it a tool for the mind to recognize reality.

The concept of straight lines was controversial and heavily influenced the politics and society of Europe from the 15th century to the 17th century. Greek thinkers and scholars like Aristotle in the 5th century already proved that the Earth was a globe, but many Europeans at that time did not believe in this idea! However, some Europeans during the Age of Exploration denied this belief. Just as the famous Italian scientist Galileo Galilei was persecuted（迫害） for advocating a heliocentric （日心的） model of the solar system, many thinkers like Giordano Bruno were shamed for believing that the Earth was round.

Well, my friends, let's move on to life. The concept of nothing going in a straight line can be associated with life as much as it relates to science and architecture. Whenever you do something, it never turns out to play out exactly as planned. I especially know that as a thirteen-year-old! Life is a rough road—every time you go forward, it is followed by two steps back or to the side. Just like how science explains it, life is a curvy path full of unexpected twists, turns, and adventures that nobody can ever predict.