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Screws
螺丝钉
Because you see screws every day, every where, you may think they're not important or great.
因为你随时随地都能看到螺丝钉，就可能认为它们不重要或不伟大了。
But screws are very useful, important and great tools.
但螺丝钉是非常有用、重要且伟大的工具。
Screws look like nails, but they have spiral shapes on their sides unlike nails.
螺丝钉看起来像钉子，但和钉子不同，它们的侧面有螺旋。
Therefore, instead of a hammer, a screwdriver is necessary to use screws.
因此，使用它时不用锤子，而用螺丝刀。
Screws are very useful to fasten parts of a machine.
螺丝钉对于固定机器零件非常有用。
Almost every machine uses screws.
几乎每台机器都用到螺丝钉。
People have used screws for a really long time.
人们使用螺丝钉很长时间了。
Greeks and Romans used wooden screws about 2,500 years ago.
希腊人和罗马人从大约2500年前就开始使用木制螺丝钉了。
Now, people use various materials to make screws.
现在，人们使用各种材料来制造螺丝钉。
Stainless steel is the most common material.
不锈钢是最常见的材料。
Plastic screws are also used for kids' toys.
塑料螺丝钉也用于儿童玩具。
