You can not see any object unless light from that object gets into your eyes. Some of the things you see give off light of their own. The sun, the stars, a lighted lamp are examples that can be seen by their own light. Such things are luminous. Most of the things you see are not giving off light of their own. They are simply reflecting light that falls on them from the sun or some other luminous bodies. The moon, for example, does not give off any light of its own. It is non-luminous. You see it because sunlight falls on it and some of it reflects in our direction. So moon light is only second hand sunlight.

When you look at a book, it sends to your eyes some of the light which falls on it, and you see the book. If light could be kept out from where you are so that there would be no light for the book to reflect, then you could not see the book even with your eyes wide open.

Light travels so fast that the time in which it travels from the book you are reading to your eyes is so short as if there were no time at all. Light reaches us from the moon, which is about 380 000 kilometers away, in only a little more than a second.

Scientists have found that migrating (迁徙) birds fly not just in groups all the time, but in “V” formation and they have also tried to find out what good points birds get from this “V” formation. Now, a research group from the University of London may have found the answer – migrating birds fly in a “V” to save energy, according to a study published in the journal Nature.

When a bird flaps (拍打) its wings, it makes the air move in different directions. Scientists found in the study that the air creates an upward-moving wave at the tips of the wings (翼尖), which means that if a bird flies around the wingtips of another bird, it can get some help from the rising air and then they can use less energy to stay in the air.

But there is a small problem – the lead bird gets no lift advantage and can easily get tired. This is why a group of migrating birds change their leaders from time to time, according to The New York Times.

Scientists were also surprised to see that migrating birds timed their wing beats and changed their places in a very simple way to make the most of the lifting effect while avoiding areas where the air moved downward.

“They’re able to sense what’s going on from the bird in front, where this ‘good air’ is coming from and how to help themselves perfectly in it,” lead researcher Steven Portugal told BBC.

In fact, the “V” formation has long been thought to help birds fly more efficiently (有效地) without wasting too much energy. Jet fighters (战斗机) were also found to be able to reduce their energy use by up to 18% by staying near the wingtips of other Jet fighters. Both of these findings led scientists to wonder that the “V” formation had an efficiency purpose, but until now they still don’t have enough facts to show what they are thinking about is really true.

“For scientists, the new study provides an insight into an interesting natural thing. But it could mean even more for airplane companies – helping them understand how they can imitate that with their plane formations to save energy,” said Portugal.

A carnival is a festival celebrated in many different ways in many countries.

New Orleans: One of the most famous carnivals in the world is in New Orleans, USA. It is called Mardi Gras. The celebrations last around two weeks, with a parade(游行) every day. Groups of people, called “krewes”, dress up in special clothes and ride on beautiful boats. They throw small gifts at the people watching. It is said that over a million people join in the celebrations every year.

Rio de Janeiro: The carnival in Rio de Janeiro, Brazil is the biggest in the world. The Rio Carnival is famous for its samba(桑巴) music and the amazing clothes and boats. People need to buy tickets to see the parades which are a kind of competition and the samba groups compete on their clothes, dancing, boats and music. Some samba schools spend millions of dollars on their preparations, but it is said that the carnival makes over $40 million from ticket sales and advertising.

Binche: The carnival in Binche, Belgium may have the longest history. Up to 1,000 men and boys, called “Gilles”, dress up in masks(面具), special clothes and wooden shoes. They also wear very tall hats, up to 90 cm tall. The Gilles dance through the streets and throw oranges at the crowd. The person getting an orange has good luck.