Today, though, around half of the world's kids have  eyeballs that are too long.

And as a result, they have blurry vision.

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Most babies are born with short eyeballs.

But as  a baby grows, their eyeballs get longer - the lens  and the retina get farther and farther apart.

By  the time the kid is about 6, 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, rather than focusing well  behind it.

At that point, the brain sends a signal  to the eyeballs, telling them to stop growing.

But, starting a few decades ago, many kids like,  more than 90% of kids in some countries, these kids' eyeballs continued to lengthen  well past that spot.

As a result, instead  of focusing light right onto the retina,  the lenses in these kids' longer eyeballs  focus light on a point in front of the retina.

From there, the light spreads back out, causing  them to see a fuzzy image rather than a crisp one. 

For years, most scientists thought this  was happening because of screens.

Or more   specifically, because kids were spending most of  their time looking at things only a short distance  away.

You see, our eyes focus most easily on stuff  in the middle distance.

In order to clearly see   stuff far away, the muscles have to work to  stretch the lens to bring those images back  he sweet spot on the retina,

and in order  to clearly see things close up, the muscles   have to work to smoosh the lens to bring those  images forward to the sweet spot on the retina.  

Scientists wondered whether kids' eyeballs were  growing extra long to shift this entire range  farther back, 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, no matter how much they strained. 

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.

Instead,   it seems that the likely culprit is the hormone  that carries the stop-growing signal from the  brain to the eyeballs – or, really, a lack of  this hormone.

We still don't totally understand   how the entire signaling-process works, 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.  

Kids today - who only spend about half as much  time outside as their parents did – simply aren't   getting the light their eyes need to create enough  of that hormone and give the stop-growing signal.  

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 .. uhh .. seen.   

Luckily, there's an easy solution for future generations  - go outside to watch your YouTube videos.