The word“day” has two meanings. When we talk about the number of days in a year,  we are using “day” to mean 24 hours. But when we talk about day and night,  we are using“day” to mean the time between sunrise and sunset. Since the earth looks like a ball,  the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转）of the earth. At the equator（赤道）day and night are sometimes the same length（长度）. They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock in the evening. For six months the North Pole is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

    What do Napoleon, Clinton and Wang Nan have in common? They are all left-handed.

Today about 15% of the people are left-handed. But why are people left-handed? The answer is the way the brain (大脑) works. The brain has two halves—the right half and the left half. The right half controls (控制) the left side of the body, and the left half controls the right side of the body. So right-handed people have a strong left brain, and the left –handed people have a strong right brain.

    The two halves of the brain are about the same size. But each side controls different things. The left side controls language, Math and logic (逻辑). When you remember new words, or when you put things in order, you use your left side.

    The right side of the brain controls color and music. It is also good at recognizing (认出)faces. This does not mean that all artists are left-handed and all accountants (会计师) are right-handed. Some right-handers have a strong right brain, and some left-handers have a strong left brain.

    Sometimes people cry when they are sad. Other times people cry tears of joy. But your eyes make tears all the time, whether you are crying or not. Do you know the tears help keep your eyes healthy? They keep your eyes from drying out. A special area of the eye drips (滴) all the time. It keeps the eye humid.

    If you look in a mirror, you can see tiny holes in the comers of your eyes. Each hole leads to a small tube that runs to your nose. Tears run slowly into this tube drip by drip. Day and night the holes drain (流走) the tears away. If they didn't, you would always look as if you were crying.

    If you begin to cry, there are many more tears. The holes can't drain all of them. Then extra tears run out onto your face.    

    Tears help keep eyes safe. If there is something harmful in the air, the eyes fill with tears. These tears coat your eyes. They keep the harmful air out.

    Contact lenses(隐形眼镜) can make the eyes too dry. Some people have to add tears to their eyes. They buy bottles of eye drops to keep their eyes wet.

    Bioplastic(生物塑料) is a kind of plastic, but it is not the same as normal plastic. Normal plastic is made from chemical materials and takes a very long time to degrade(降解). But bioplastic is good for the environment. It is made by using bacteria(细菌) to process(加工) natural sources like vegetable oils, corn or sugar. It can degrade more easily.

    But there is a problem with bioplastic. The "food" that bacteria eat is what we eat. This means that bacteria are competing against humans for the same resources.

    Now scientists have found a new way to make bioplastic——using two kinds of bacteria and sunlight, according to Newsweek.

    One of the bacteria can use sunlight to produce sugar. The other kind lives in the salty waters of Bolivia. It eats the sugar that the first bacteria produce and then it can produce polymers(聚合物). Polymers are used to create bioplastics. The way the bacteria store polymers is similar as humans store fats.

    The new way to produce bioplastic has many advantages. It is 20 times faster than the normal way we produce bioplastic. Moreover, using these two bacteria saves resources.

    Firstly, producing a lot of bioplastic requires corn and sugar, which are used to feed normal bacteria. And growing corn and sugar needs farmland. But these two bacteria can just use sunlight. The land that is saved can be used to grow food for humans. Besides, this system doesn't need rich land or clean water at all, scientists said to Newsweek.

    The next problem scientists face is that they want to put their lab research into real-world practice. By doing this, bioplastic made with these two bacteria can be widely produced in the future.