Have you ever heard someone use the phrase"once in a blue moon"？People use this expression to describe something that they do not do very often．For example，someone might say that he tries to avoid eating sweets because they are unhealthy，but will eat chocolate"once in a blue moon"．Or someone who does not usually like to go to the beach might say"I visit the shore once in a blue moon．"While many people use this phrase，not everyone knows the meaning behind it．

    The first thing to know is that the moon itself is never really blue．This is just an expression．In fact，the phrase"blue moon"has to do with the shape of the moon，not the color．

    As the moon travels around the earth，it appears to change shape．We connect names with certain shapes of the moon．For example，when we can see a small part of the moon，it is called a crescent moon．A crescent is a shape that looks like the tip of a fingernail（指甲尖）．When we cannot see the moon at all，it is called a new moon．When we can see the whole moon，it is called a full moon．Usually，there is only one full moon every month．Sometimes，however，there are two full moons in one month．When this happens，the second full moon is called"a blue moon"．

    Over the next 20years，there will only be 15blue moons．So you can seldom see a blue moon． This fact has led people to use the expression"once in a blue moon"to describe things they don't often do in their lives．

    There are some easy things you can do to protect the environment and the earth. Choose ideas from the list or come up with a few of your own.

    ·Plant flowers, grass or trees.

    ·Whenever you visit a park or beach, take away what you bring there-keep rubbish in a bag until you can put it in a dustbin.

    ·Turn off the lights, TV sets when you leave the room. This saves a lot of electricity.

    ·Turn off the tap when you brush your teeth. You can save some water by not letting it run. Also, use a glass cup instead of a paper cup because this saves paper.

    ·Keep the doors and windows closed in winter to keep cold air in.

    ·Give your old books and magazines to a library instead of throwing them away.

    ·Give your old clothes to poor children you know instead of throwing them away.

    ·Use both sides of paper.

    ·Stop pouring dirty water into the rivers or lakes nearby.

    ·Encourage all your friends to do the same things you do to help protect the earth.

    You don't have to wait until Earth Day to do these things. Make every day Earth Day.

    If everyone makes a contribution to protecting the environment, the world will become much more beautiful.

    A thirsty crow(乌鸦) comes across(偶然遇到) a bottle of water, but the water level is out of its reach. The bird then drops some stones into it and gets the water.

    "It's just a story," you may think. But new research has found that crows' brains may sometimes be better than those of 6­year­old children.

In a recent experiment, American scientist Corina Logan and her team caught six crows to test them. The crows and children had to choose between two sets of tubes(试管). They had to drop stones in and get water.

    With the red set, when they dropped a stone into a wide tube, the water level raised in a connected narrow tube. The blue set of tubes, however, had no connection between them.

    Children aged 7 to 10 were able to learn the rule. Children aged 4 to 6, however, failed. Five of the six crows failed the test. But Kitty, a 6­month­old crow, passed it. She put most of the stones into the red tubes.

    So, could a bird be more clever than a kindergartner? Don't be too quick to say no.

    Some of the greatest problems we face today are the destruction (破坏) of our environment. Brown clouds, polluted water, endangered wild animals…. these problems seem so huge.

    So my family does what we can. We take cloth bags to stores instead of using plastic bags. We walk where we don't have to drive…

    But does it do any good? When I am the only one in line at the market with cloth bags, am I doing any good? Does my walking to stores make any real difference to the world?

    I recently learned something about flamingos(火烈鸟) which like to get together in groups of a thousand or more. Every year, when the time comes for migration(迁徙), a few of them first take off from the lake. But none of the others seem to notice. So the small group returns. However, the next day they try again. This time a few more fly along with them, but most of them still pay no attention, so they return again. They try for several times. Every time a few more birds join in but, since thousands of the others still take no notice, the great migration plan is once more stopped.

    Then one day something changes. The same small group of birds once again starts flying and a small number more join in just as before, then more. Finally, they all take flight and the migration really begins. What a spectacular sight it must be — thousands of flamingos taking off into the sky at once!

    A few can make a difference. Even if you're the one to take the first step, and continue trying, others will someday take notice and together we will solve even our greatest problems.