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Humans may be no less a part of the animal kingdom than lions, tigers and bears, but certain things set us apart. For instance, although like our mammalian cousins we use teeth to bite and chew food, we’re the only animals that brush after every meal.
或许人类的动物属性一点也不比狮子、老虎和熊少，但某些特征却将我们与它们区别开来。举例来说，虽然我们与其他哺乳动物一样用牙齿撕咬和咀嚼食物，但我们是唯一餐后刷牙的动物。

Brushing
刷牙

And as four out of five dentists have reminded us on countless¹ television commercials, using one or another variety of toothpaste is crucial for keeping our teeth healthy.
正如五个牙医中会有四个在数不胜数的电视商业广告中一直提醒我们的那样，使用不同种类的牙膏对保持牙齿健康至关重要。

Like that fifth dentist who never seemed to know what was going on, you might have wondered what toothpaste actually is, and how it helps ward² off tooth decay.
像永不情知的的第五个牙医一样，你可能会疑惑，牙膏到底是什么，它又如何帮助我们避免龋齿。

What Is In Toothpaste?
牙膏里有什么？

Toothpaste consists of several ingredients, some of which you’d never think to put in your mouth. For example, almost all toothpaste contains detergent³, or soap. Getting your mouth washed out with soap is supposed to be punishment, so what is it doing in toothpaste?
牙膏含有多种成分，有一些你可能从未想过要把它们放进嘴里。比方说，几乎所有的牙膏都含有清洁剂或肥皂。直接用肥皂清洁嘴巴是一种惩罚，那么牙膏中含有的肥皂是如何清洁牙齿的呢？

Although it has a slight antibacterial effect, it’s there mainly to make foam⁴, which serves little purpose other than to provide a visual sign to the brusher that the toothpaste is hard at work fighting cavities.
 尽管肥皂有轻微的抗菌作用，但它主要是制造泡沫，这些泡沫除了向刷牙的人传递牙膏能有效抗击龋齿的视觉信号外毫无作用。

Ingredients
成分

Of course some ingredients play a more active role, the most famous of which is fluoride, a mineral compound derived⁵ from the element fluorine. Fluoride protects teeth in two ways: it kills acid-producing bacteria which are mainly responsible for tooth decay, and it strengthens and repairs tooth enamel⁶ that has been softened⁷ by the acid.
当然牙膏里有一些成分在发挥着更为积极的作用，当其中最广为人知的还是氟化物，这是一种从氟元素中提取出来的无机化合物。氟化物以两种方式保护牙齿：首先，它能够杀死龋齿元凶-造酸细菌；另外，它还可以固化、修复被酸软化的牙釉质。

So far as we know, the only drawback to using toothpaste is that it makes drinking orange juice after brushing a pretty unpleasant taste experience. But that’s a small price to pay for healthy teeth.
据我们所知，使用牙膏的唯一缺点在于：它使人们刷牙后饮用橙汁成为一种相当不悦的味觉体验。但这只是为牙齿健康付出的一点小小代价。