This week we've got a question from Lulu Miller, the host of NPR's Invisibilia podcast:

本週我們收到美國公共廣播電台《探索未知》節目主持人 Lulu Miller 提出的一個問題：

How old are our bodies actually?

我們身體的真實年齡是幾歲呢？

Because if you think about how we're constantly being regenerated our physical matter is being shed and replenished... what actually remains?

如果你想一想我們的身體是如何不斷地再生，我們身體細胞的損耗和重生...什麼是持續存在的呢？

Good question, Lulu!

Lulu 問得好，這是一個很好的問題！

Well, your hair obviously doesn't remain the same and I'm not just talking about the style.

嗯，你的頭髮一定不會永遠不變，我的意思並不是說 「髮型 」會變。

Each hair on your head is replaced every 2 to 7 years.

而是說，你頭上的每根頭髮每 2-7 年會更換一次。

A hundred hairs fall out every day and new ones grow back in their place.

我們每天有數百跟頭髮掉落，新的頭髮也會在原本的位置重新長出來。

And look at your fingernails - they're completely new every six months or so.

看看你的指甲 - 他們大約每 6 個月會完全更新一次。

It turns out it's just a matter of time before almost every part of your body refreshes itself in a similar way.

也就是說，只是時間長短的問題，幾乎身體的每個部分都會損耗和再生。

The lining of your stomach and intestines gets pretty beat up -- it's constantly exposed to acid and bile and so those cells get replaced every few days.

你的胃和小腸的內壁細胞被攻擊得非常厲害 - 他們一直暴露在酸和膽鹽的環境中，所以這些細胞每隔幾天就會更新一次。

Every few weeks, your outer layer of skin is completely renewed.

每隔數週，你的皮膚外層細胞會全部更新一次。

Every four months, you have a fresh army of red blood cells.

每 4 個月，你會有一批新的紅血球軍團。

A hundred million new cells are born every minute and a hundred million old cells are destroyed.

每分鐘有上億的新細胞誕生，同時也有上億的細胞死亡。

It's actually the breakdown products of these red blood cells that turn your bruises and urine yellow.

你身上的挫傷和尿液的黃色就是來自這些紅血球細胞的分解產物。

Every 10 years, you've got a new skeleton.

每 10 年，你會有一個全新的骨骼。

A special team of cells breaks down old bone, and another builds new bone.

有一群特殊的細胞負責把老骨頭分解，也有另外一群細胞會建造新的骨頭。

Every 15 years, your muscles are refreshed.

每15年，你的肌肉會再生。

You might think you gain and lose fat cells when you gain and lose weight but they actually just get bigger and smaller.

你也許會以為體重的增加和減少是因為脂肪細胞數目的增加和減少，但其實體重變化是和脂肪細胞變大和變小有關。

Over the course of 25 years though, most of them turn over.

在 25 年的時間內，大部份脂肪細胞都會更新。

But there are a few things that stick around for your entire life.

但是，也有幾件事情是一生都不會變的。

About half of your heart stays with you from birth to death because those cells are replaced very slowly.

像是一半的心臟細胞從你出生到死亡都會跟著你，因為它們的更新率非常慢。

Certain parts of your brain add a few new neurons over the course of your life, but the vast majority of your neurons developed before you were born.

在你的一生當中，你的某些腦區塊會增加幾個新的神經元細胞，但是大部分的腦神經元細胞是在你還沒有出生前就生成的。

It's the connections between those neurons -- the circuits that store memories that are constantly changing.

而負責流通傳遞儲存記憶，連接這些神經元的膠質細胞則會不斷地改變。

And there's one more part of you that lasts your whole life.

還有一個部份是會跟著你的一生的。

Months before you were born, a little cluster of cells stretched and filled themselves with transparent protein.

在你出生的數個月前，一小群細胞會利用透明蛋白延展和充填自己。

As you grew, even after birth, more and more fibers were added, but that center endured.

隨著你的成長，甚至是在出生之後，更多更多的纖維會加入，但是那個核心部分是不變的。

This is your lens, the window through which you are watching this video right now, and its core has remained the same since the moment you first opened your eyes.

這個就是你現在用來看影片的眼球晶體，從你一睜開眼開始，它的核心部分一直沒有改變過。

So how old is your body really?

所以，你的身體到底幾歲呢？

Some of it is brand-new and some of it is as old as you are.

有些部分是全新的，而有些部分則和你的年齡相同。

To learn how scientists figured all this out, check out our post at skunkbear.tumblr.com

想了解科學家如何發現這些事情，請點閱：skunkbear.tumblr.com。

And listen to Invisibilia -- the latest episode is about whether or not personality persists throughout your entire life

以及收聽《探索未知》節目，最新的一集是探討在一生之中，你的人格是不是永恆不變的呢？

If you have science questions send them our way and please subscribe to our channel!

如果你有科學方面的問題，請寄給我們，也請訂閱我們的頻道！