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Squeaks and I love learning more about how our bodies work,
我和吱吱喜欢更多地了解身体是如何运作的，
and he just asked me a great question: Where does gray hair come from?
他刚问了我一个好问题：白发从哪儿来？
Lots of grown-ups have gray hair, but it wasn't always gray.
许多成年人都有白发，但它并不总是灰白色的。
Everyone who has gray or white hair used to have hair that was a different color, like blonde or brown, or red.
白发的人过去曾有不同的发色，如金色、棕色或红色。
Hair turns gray as people get older, because the stuff that gives their hair its color starts to run out.
头发随着人变老而变得灰白，因为为头发提供颜色的物质开始用完了。
Each hair we have grows from its own little spot under our skin, called a follicle.
我们的每根头发都是由皮肤下它自己的毛囊生长出来的。
A follicle is like a tiny hair-making factory,
毛囊就像一个微型的头发生产工厂，
putting together fibers to make the strand of hair and adding color to it.
它将纤维组合成一缕头发，并为它添加颜色。
That color comes from something called melanin, which is kind of like hair dye that our bodies make by themselves.
头发的颜色来自于一种叫做黑色素的物质，有点儿像我们身体自己制作的染发剂。
That's why people's hair can be lots of different colors, it just depends on the types of melanin they have.
这就是人们的头发可以有很多不同颜色的原因，这取决于他们的黑色素类型。
We have melanin in other parts of our bodies, too.
我们身体的其他部位也有黑色素。
It gives color to our skin, and even our eyes!
它为我们的皮肤甚至眼睛染色！
There are lots of little tiny things inside hair follicles, called melanocytes, that are always making more melanin.
毛囊里有很多微小的物质，叫做黑色素细胞，它们总能产生更多的黑色素。
Every day, the follicle adds more fibers to the bottom of the hair, so the strand gets a little longer.
每天，毛囊会在头发底部增加更多的纤维，所以发丝变长了。
At the same time, it uses melanin from the melanocytes to color the hair.
同时，它利用黑色素细胞中的黑色素来给头发染色。
That's how our hair grows! But each hair doesn't grow forever.
我们的头发就是这样生长的！但头发不会永远生长。
After a few years, it falls out, and the follicle starts over with a new hair.
几年后，它就会脱落，毛囊开始产生新的头发。
Now, that doesn't happen to all of our hair at the same time,
当然，这不是同时进行的，
only a few hairs fall out and start regrowing every day, which is why you might not even notice it happening.
每天仅有几根头发脱落然后再重新生长，这就是你可能没注意到的原因。
Every time the hair falls out, the follicle also loses a few of the melanocytes that were making the melanin.
每次掉发，毛囊也会失去一些生成黑色素的黑色素细胞。
Our bodies can make enough melanocytes to keep growing and coloring new hairs for a very long time.
我们的身体很长一段时间内能够制造足够的黑色素细胞，来保持新头发的生长和染色。
But after lots of years of growing new hairs, the follicles start to run out.
但经过多年新发的生长，毛囊开始耗尽。
They can still make the hair itself, but without melanocytes,
它们仍能够产生头发，但没有黑色素细胞，
they can't make the melanin to color it anymore.
它们就不再产生黑色素来染色了。
And guess what the hair looks like without the color?
那么猜猜没有颜色的头发是什么样子？
Exactly! Without the color from melanin, hair is gray or white.
答对了，没有黑色素的头发是灰白色的。
At first, just a few hair follicles run out of melanin, so the person only has a few gray hairs.
起初，只有少数毛囊用光了黑色素，所以这个人只有几根白发。
As more and more follicles run out of color over time, they get more gray, until eventually all their hair is gray.
随着时间的推移，越来越多的毛囊用光了黑色素，头发变得更白了，直到最后全白。
You probably have a long time before your hair goes gray, though.
不过，你可能需要好久头发才变白。
Everyone's different, but people usually get their first few gray hairs when they're around 30 or 40 years old,
每个人都不一样，但人们通常在30、40岁的时候就会有几根白发，
and most of their hair is gray by the time they're 50 or 60.
到50、60岁时大部分都变白了。
Some people decide to dye their hair a different color when it starts turning grey
一些人决定在头发开始变白时把头发染成不同的颜色，
but lots of people keep it grey or white.
但许多人仍留着灰白的头发。
So, Squeaks, there's your answer.
所以，吱吱，这就是你的答案了。
Going gray is just a normal part of getting older!
头发变白是变老的正常表现！
Thanks for joining us!
感谢您的收看！
If you want to keep learning and having fun with Squeaks and me, be sure to subscribe,
如果你想继续学习或与我和吱吱共度美好时光，就一定要订阅哦！
And we'll see you next time, here at the Fort!
我们下期见，沃斯堡等你哦！
