The Ushaped pipe under a sink，tub，or toilet is called a “water trap”．The waste water that goes down the sewer gives off a smelly gas called sewer gas．When waste water drains through a water trap，some of the water stays in the bottom curve of the pipe．The trapped water forms a water seal that keeps the sewer gases from coming back into the house．
为什么洗涤槽下面的管子是弯曲的
在洗涤槽、浴盆或便池下面的U形管子叫“存水湾”。流往排水管的废水散发出叫做污水气的臭味。当废水通过存水湾排出时，一些水就留在管子底部的弯曲处。存下的水形成了一个水封门，可以防止污水气返回到房子中来。
How Does the Sun Tan Our Skin
There are special cells in your skin that produce a dark colouring pigment.This pigment，called melanin，gives the brown colour to the skin．When your skin is exposed to sunlight，the melaninproducing cells make an extra amount of melanin，and the skin becomes darker than usual．We call this browning action“tanning”．
太阳是怎样把皮肤晒黑的
你的皮肤里有一种特别的细胞，这种细胞能产生黑色素。这种色素即黑色素能使皮肤变成棕色。当你的皮肤曝晒在阳光下时，制造黑色素的细胞产生出过量的黑色素，从而皮肤就变得比平时要黑。我们把这种黝黑现象称为皮肤晒黑。
What Makes a Boomerang Return
The returning boomerang is nearly Vshaped，and its arms are slightly twisted．The thrower hurls the boomerang forward a flip of the wrist so that it Spins through the air．Because of its twisted shape，the air pushes against one part of the boomerang more than another，so that it has to travel in a curved path．As it slows down，the boomerang circles and returns toward the thrower．
是什么使飞镖返回的
能返回的飞镖近似V字形，它的两臂稍有点扭曲。投掷者用腕关节轻轻一弹把飞镖往前投去，使得飞镖在空中旋转。由于飞镖的旋扭形状，空气对飞镖一边的推动力大于另一边，使得飞镖势必呈弯曲状运行。当飞镖慢下来时，它盘旋着，并朝着投掷者的方向返回。
How Are Baseballs Made
When you play with a baseball you are playing with a ball of yarn that has a leather cover glued around it to keep it from unwinding．To make an official baseball，the ball maker winds yarn around a small cork and rubber core．When it is nearly the size of a baseball，the ball maker dips the ball of yarn into a tub of special glue and stitches a leather cover over it．
棒球是怎样制成的
当你玩棒球时，实际上你是在玩纱线球，这是一种为了防止纱线散开而粘贴着一层皮革面的纱线球。为了制作一个正式的棒球，制球工人在小块软木和橡皮芯上缠绕纱线。当缠绕到接近一个棒球那么大时，工人就把纱线球浸入一个装有特种胶水的容器中，然后在其表面上缝一层皮革。
What Causes the Colour We See
Light is made up of colours．Wherever light exists，colour does，too．The colour of any object depends on the colour of the light rays it reflects to our eyes．An apple looks red because it reflects the red rays of light．The ocean looks blue because it reflects the blue rays．Where there is no light，there is no colour．A tree that is green when we see it in light appears black at night．
我们所看到的颜色是什么引起的
光线是由各种颜色组成的。哪里有光，哪里也就有颜色。任何物体的颜色取决于反射到我们眼中的光线的颜色。一个苹果看上去是红的，那是因为它反射出红色光线。海洋看上去是蓝色的，那是因为它反射出蓝色光线。哪里没有了光，哪里也就没有颜色。一棵树，当我们在光线下看时，它是绿的，而在夜里，它却变成黑色。
Why Does a Train Have a Caboose
The caboose is the end car on a freight train where supplies and signals are kept．It is where the brakeman rides while the train is moving．He watches out of the caboose’s large windows to see that the train is running smoothly．It also serves as an off the on wheels for the train’s conductor．Here he keeps records of the freight，and where to drop off cars and pick up other cars．
为什么火车有一节守车
守车就是一列货车的最后一节车厢。在这节车厢里，存放着供应品并提供信号。当火车运行时，司闸员就乘坐在这里。他从宽大的守车窗户向外观看列车是否正在顺利运行。这节车厢也作为列车员行车时的办公室。在这里列车员记载着装车的货物及该在什么地方把一些车厢脱钩，把另外一些车厢挂钩。
Where Does Chocolate Come From
Chocolate is made from the seeds，or beans，of the tropical cacao tree．The beans grow in pods that look like long cantaloupes．At the chocolate factory，the beans are roasted and ground．The grinding action melts the fatty“cocoa butter”in side the beans，and turns it into a thick chocolate liquid．When poured into moulds，the liquid hardens into cakes of unsweetened chocolate．
巧克力是从哪里来的
巧克力是用热带可可树的籽，即可可豆做成的。可可豆长在豆荚里，这种豆荚很像长长的罗马甜瓜。在巧克力加工厂里，可可豆被烘烤和磨碎。在碾磨过程中，溶化了可可豆内所含的脂肪“可可脂”，并使它成了巧克力浓液。当浓液注入模子后，浓液变硬，就成为一块一块没加糖的巧克力。
Why Does Silver Tarnish
Silver tarnishes for almost the same reason that iron gets busty．Tarnish is a form of corrosion．It is caused when oxygen in the air chemically combines，or joins，with the silver metal，covering it with a darkcoloured film．The sulfur in such foods as eggs and cabbage also causes sliver to quickly tarnish．To make silver look shiny again．we must use a sliver polish to remove the tarnish．
银器为什么会失去光泽
银器失去光泽几乎和铁生锈是同样道理。失去光泽是腐蚀的一种形式。当空中的氧与银化合时，上面就覆盖一层黑色的薄膜，这就使银器失去了光泽。某些食物如鸡蛋和包心菜中的硫磺，也能使银器很快失去光泽。要想使银器重新光亮，我们必须用银器擦光剂把锈蚀去掉。
What Are Mummies
A mummy is simply a body that has been preserved by drying．The Egyptians believed that the spirits of the dead would live forever if their bodies were preserved．To mummy a body，the Egyptians first dried the body with chemicals，then wrapped the dried body in layers of cloth．The oldest mummy known to exist is that of the Egyptian Pharoah，RamsesⅡ．It is over 3，200years old．
什么是木乃伊
木乃伊只不过是用干涸的办法保存起来的尸体。埃及人相信，只要人的尸体能保存下来，死人的灵魂将会永远活着。为了将一具尸体制成木乃伊，埃及人先用化学药品将尸体干涸，然后把干尸缠上一层一层的布。现在所知道的最老的木乃伊，是埃及法老雷姆塞斯（Ramses）二世的木乃伊，它已有3200多年的历史。
How Does a Scab Form
When you cut yourself，blood flows out of the wound．You don’t bleed for a long time because the blood thickens，or clots，and fills the cut．Little by little，the clot hardens until it forms a tough scab．The scab protects the cut while it is healing．When the cut is completely healed，the scab falls off．
痂是怎样形成的
当你身上什么地方被割破时，血就从伤口流出。但是，血并不流很长时间，因为血会浓缩或凝固，并堵住了伤口。然后，凝块逐渐变硬，直到形成一个坚硬的痂。在伤口愈合时，痂保护着伤口。当伤口完全愈合以后，痂便脱落。
How Is Dry Milk Made
Dry，or powdered，milk is made by evaporating the water from the milk，and only the dry milk solids are left．To dry milk，the milk is heated to evaporate some of the water．The thickened milk is then sprayed as a fine mist into a drying chamber，where drafts of hot air quickly evaporate the remaining water in the milk，leaving a fine powder．Dry milk can be kept a long time without spoiling．When water is added，dry milk is as nourishing as fresh milk．
奶粉是怎样制成的
奶粉的做法是把牛奶里的水蒸发掉，使之剩下干奶粉。要制奶粉，就要把牛奶加热，蒸发掉一些水分，然后把变浓的牛奶像喷细雾一样地喷进烘干箱，在那里热气流很快地把牛奶里剩余的水分蒸发掉，而只留下细粉末。奶粉能长期保存而不坏。当用水冲奶粉时，它又像新鲜牛奶一样富有营养。
What Book Is the Statue of Liberty Holding