which is too small to be seen but roomy enough to hold thousands of complicated structures like mitochondria, 小到几乎看不见，但大得足以容纳数以千计的像线粒体这样的复杂结构，

and millions upon millions of molecules. 以及几百万几百万个分子。

In the most literal way, cells also vary in liveliness. 从最基本的方面来说，细胞的活力也各不相同。

Your skin cells are all dead. 你的皮肤细胞都是死的。

It's a somewhat galling notion to reflect that every inch of your surface is deceased. 想到自己身体表面的每一部分都是死的，你也许会感到有点屈辱。

If you are an average-sized adult you are lugging around about five pounds of dead skin, 如果你是个中等个儿的成年人，你身上裹着大约2千克的死亡皮肤，

of which several billion tiny fragments are sloughed off each day. 其中每天都有几十亿的微小组织从你身上脱落。

Run a finger along a dusty shelf and you are drawing a pattern very largely in old skin. 如果你将一个手指从布满灰尘的搁架上划过，那个痕迹在很大程度上是用你死去的皮肤划成的。

Most living cells seldom last more than a month or so, but there are some notable exceptions. 大多数细胞的存活时间很少超过l个月左右，但也有一些明显的例外。

Liver cells can survive for years, though the components within them may be renewed every few days. 肝脏细胞可以存活几年，虽然它们的内部成分每隔几天就更新一次。

Brain cells last as long as you do. 大脑细胞和你的寿命一样长。

You are issued a hundred billion or so at birth, and that is all you are ever going to get. 从你出生起，你拥有大约1000亿个细胞，这也就是你所能拥有的细胞数的最高值。

It has been estimated that you lose five hundred of them an hour, 据估计，你每小时大约丢失500个细胞。

so if you have any serious thinking to do there really isn't a moment to waste. 因此，要是你认真想一想的话，你真的是一刻光阴也不该浪费。

The good news is that the individual components of your brain cells are constantly renewed 令人欣慰的是，你脑细胞的组成部分总是在不断更新，

so that, as with the liver cells, no part of them is actually likely to be more than about a month old. 因此。与肝脏细胞相类似，你的大脑细胞实际上只存活1个月左右。

Indeed, it has been suggested that there isn't a single bit of any of us—not so much as a stray molecule—that was part of us nine years ago. 事实上，据认为，我们身上的任何一个部位——包括迷途分子在内——都与9年前不同。

It may not feel like it, but at the cellular level we are all youngsters. 这听起来似乎有些玄乎，但从细胞的层面上讲，我们都是年轻人。