（滑動查看雙語字幕）To be able to see things clearly, your eyeballs need to be a certain shape.為了能夠看清，你的眼球需要有一定的形狀。 Today, though, around half of the world's kids have eyeballs that are too long.然而，如今世界上，大約有一半的孩子的眼球都很長。 And as a result, they have blurry vision.所以，他們的視力會很模糊。 Welcome to Minute Earth.歡迎來到今天的一分鐘科普時間。 Most babies are born with short eyeballs.大多數嬰兒出生時眼球都很短。 But as a baby grows, their eyeballs get longer.但是隨着不斷成長，他們的眼球會變長。 The lens and the retina get farther and farther apart.晶狀體和視網膜的距離越來越遠。 By the time the kid is about 6,當孩子大約6歲的時候， their eyeballs are just the right length for the lens to be able to focus incoming light and form a crisp image right on the retina,他們的眼球的長度正好使晶狀體能夠聚焦入射光，並在視網膜上形成清晰的圖像， rather than focusing well behind it.焦點不會挪到視網膜後方去。 At that point, the brain sends a signal to the eyeballs, telling them to stop growing.此時，大腦會向眼球發出信號，告訴它們停止生長。 But, starting a few decades ago, many kids, like, more than 90% of kids in some countries.但是，從幾十年前開始，一些國家的許多孩子，準確說，大概超過90%的孩子。 These kids' eyeballs continued to lengthen well past that spot.這些孩子的眼球會繼續生長，離最佳形狀越來越遠。 As a result, instead of focusing light right onto the retina,因此，與其將光線直接聚焦到視網膜上， the lenses in these kids' longer eyeballs focus light on a point in front of the retina.這些孩子較長的眼球會將光線聚焦在視網膜前面的一個點上。 From there, the light spreads back out, causing them to see a fuzzy image rather than a crisp one.然後，光線會向外擴散，這樣他們看到的圖像就很模糊。 For years, most scientists thought this was happening because of screens.多年來，大多數科學家認為這是因為電子屏幕的影響。 Or more specifically, because kids were spending most of their time looking at things only a short distance away.或者更具體地說，因為孩子們大部分時間都在盯着近距離的物體看。 You see, our eyes focus most easily on stuff in the middle distance.我們的眼睛最容易聚焦在中等距離的東西上。 In order to clearly see stuff far away,為了清楚地看到遠處的東西， the muscles have to work to stretch the lens in order to bring those images back to the sweet spot on the retina.肌肉必須伸展晶狀體，才能將這些圖像映射到視網膜的最佳位置上。 And in order to clearly see things close up,為了能清楚地看到近距離觀察的情況， the muscles have to work to smoosh the lens to bring those images forward to the sweet spot on the retina.肌肉必須使晶狀體變窄，才能將這些圖像前移到視網膜上的最佳位置上。 Scientists wondered whether kids' eyeballs were growing extra-long to shift this entire range farther back.科學家們想知道，孩子們的眼球是否長得過於長了，從而將整個視線最佳範圍往後移了。 Allowing them to see close up stuff in focus without having to use their muscles, but leaving their eyes unable to focus on things far away at all,也就是說，他們更容易看清楚近距離聚焦的東西，但眼睛無法聚焦到遠處的東西， no matter how much they strained.不管肌肉是否收縮。 But recently, we've found that kids who spend a lot of time parked in front of a screen don't necessarily have longer eyeballs than those who don't.但最近，我們發現，長時間坐在屏幕前的孩子並不一定比不看屏幕的孩子的眼球長。 Instead, it seems that the likely culprit is the hormone that carries the stop-growing signal from the brain to the eyeballs.相反，可能的罪魁禍首似乎是荷爾蒙，它將停止生長的信號從大腦傳遞到眼球。 Or, really, a lack of this hormone.也就是說，是荷爾蒙缺乏導致的。 We still don't totally understand how the entire signaling-process works.我們仍然無法完全理解整個過程是如何進行的。 But we do know that our eyes need to be exposed to a certain level of light in order for the hormone to form in the first place.但我們確實知道，我們的眼睛需要暴露在一定水平的光線下，以便形成荷爾蒙。 Kids today, who only spend about half as much time outside as their parents did,今天的孩子，他們在戶外的時間只有他們父母的一半左右， simply aren't getting the light their eyes need to create enough of that hormone and give the stop-growing signal.他們的眼睛根本沒有得到所需的光線來產生足夠的荷爾蒙，並發出停止生長的信號。 As a result, their eyeballs keep lengthening, past the sweet spot, creating an epidemic of blurry vision the likes of which the world has never seen.結果，他們的眼球不斷加長，超過了最佳點，造成了一種世界上從未見過的視覺模糊的流行病。 Luckily, there's an easy solution for future generations.幸運的是，對於子孫後代來說，有一個簡單的解決方案。 Go outside to watch your YouTube videos.那就是下次你想看視頻的時候，出去看！
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