A man sat at an underground station in Washington, D.C., and started to play the violin. It was a cold day in January. He played six Bach（巴赫）pieces for about 45 minutes. It was rush hour and thousands of people went through the station, most of them on their way to work.

    Three minutes went by and the first person, a middle-aged man, noticed there was a musician playing. He slowed down and stopped for a few seconds and then hurried up to meet his schedule.

    After a minute, the violinist received his first dollar tip: a young woman threw the money in the box and continued to walk. Then another few minutes went by and someone stood against the wall to listen to him, but the young man looked at his watch and started to walk again. Clearly he was late for work.

    During the musician's performance, only six people stopped and stayed for a while, twenty gave him money but continued to walk. When he finished playing, no one applauded（鼓掌）. No one knew the violinist was Joshua Bell, one of the best musicians in the world.

    This is a real story. Joshua Bell playing in the underground station was organized by The Washington Post（《华盛顿邮报》）as part of a social experiment. One of the conclusions from this experiment could be: If we do not have a moment to stop and listen to one of the best musicians in the world playing the best music, how many other things are we missing?

    Ferdinand Magellan(1480~1521) was a Portuguese explorer. In 1519, he led the voyage to sail around the world. It was also the first voyage to sail from the Atlantic Ocean to the Pacific Ocean. Magellan gave the Pacific Ocean its name, which means "peaceful sea" in Portuguese. However, Magellan was killed in a battle before the voyage was finished. Only one of his ships, Victoria completed the journey.

    It's said that Laozi lived in the about 5th century BC. It is believed that he wrote Daodejing. This book is popular in both the East and the West, and it is still widely read today. Many people have found inspiration(灵感)from this book. There are many famous sayings in Daodejing. One of them is, "A journey of a thousand miles begins with a single step."

    Sir Isaac Newton was an English physicist, mathematician and astronomer. He discovered the law of universal gravitation. In 1687, Newton published one of the most important books in the history of science. In the book, Newton described many laws that show how the universe works. It's said Albert Einstein kept a picture of Newton on the wall of his study.

Astronauts would face dangers both getting to and surviving (生存) on Mars. Let's learn about surviving a trip to Mars.

What would it take to keep people safe and healthy on a trip to Mars? Scientists are already working on it, though the trip may be many years later.

Over the last 60 years, many spaceships have flown by orbited（沿轨道运行）and even landed on the Red Planet. However, machines cannot replace humans. That's why the United States, China and other countries want to send people to Mars. But surviving on this trip would not be easy.

A trip to Mars would be the farthest journey in human(人类) history. Mars is about 225 million kilometers away, so it would take astronauts at least 180 days to reach there. However, it only takes a few days to get to the Moon.

Space travel is dangerous. For one thing, staying in space for long periods makes the human body weak. Also, it puts pressure behind the eyes and causes eye problems. But gravity(重力) machines could help.

Then there's space radiation(辐射) to worry about. It is bad for astronauts when they have long journeys. Scientists are working out the issue. They have found that taking certain vitamins can help.

Astronauts will have to pack light to lift off from the earth. But they won't be able to get the things they need easily. Astronauts on the space station practice for this by growing lettuce and other food in space. Engineers are developing 3D printing techniques（技术）which can allow future Mars astronauts to build what they want.