This episode of SciShow Kids is brought to you by Child and Teen Checkups.

Every kid needs a checkup at least once a year. If you live in Minnesota, Child and Teen Checkups can help.

Learn more at GetCTC.com. ♪. Squeaks, want to try something cool? [Squeaks squeaks]. I have this big chart here with lots of the same letter: E.

But the Es are all pointing in different directions! All you have to do is stand back from the chart a little and tell me which direction the letters are pointing in, and we’ll know a lot about how your eyes are doing based on which letters are too small for you to see. [Squeaks squeaks]. What does the test tell us about your eyes?

Well, sometimes people — or even robot rats — have trouble seeing things clearly. The world might look kind of blurry, especially things that are far away. It can happen so slowly that you might not even notice that things are getting blurrier, so doctors sometimes do tests like this E chart to check.

I went to visit my eye doctor the other day and did a test a lot like this, except it had a bunch of different letters of the alphabet instead of Es in different directions. So I thought it might be fun to try this test at home just to show you what it’s like. [Squeaks squeaks]. Oh, Squeaks wants to know what happens if you can’t see some of the Es?

That’s totally OK. It just means you might need glasses to help you see better. Lots of people wear glasses!

There are also lots of people who wear contact lenses, which do the same thing as glasses, except they’re small and go right on top of the person’s eyeball. Glasses help the world look less blurry by changing the light you see before it gets to your eyes. It all has to do with the lens, the part of the eye right underneath the colorful part.

Its job is to turn the big wide world around you into a clear, tiny picture in the back of your eye. That’s where you have the part of your eye that sends messages to your brain about what you’re seeing, and that’s called the retina. The lens is supposed to bend the light rays so what you’re seeing becomes a small, clear picture that lands exactly on your retina.

When the world looks blurrier than it should, that’s usually because the lenses in the person’s eyeballs are shaped a tiny bit differently. If the shape of the lens isn’t completely 100% perfect, the light rays won’t get bent into a clear picture when it gets to your retina. Instead, the picture will be blurry.

You can get an idea of how this works by asking a grownup to help you shine a bright light through a magnifying glass. Don’t look at the light through the magnifying glass, though, or you could hurt your eyes. Just point the light toward something, like this desk here, and move the magnifying glass back and forth.

See how the point of light on the desk changes? The magnifying glass bends the light rays just like the lens in your eye does. If you hold the glass in a certain spot, the circle of light is small and clear.

But if you move it back and forth, the light rays bend differently, and the circle becomes blurrier. When someone’s lens is shaped a way that bends the light a little differently, the same thing happens to the picture in the back of your eye — it becomes blurrier. But the good news is, glasses can help fix the way the light bends!

Like this magnifying glass, glasses and contact lenses bend the light a little bit before it even gets to the lens in your eyeball, so by the time the picture hits the back of your eye, it’s perfectly clear. So, Squeaks, should we try the test and see how your eyes are doing? [Squeaks squeaks]. OK, so first let’s cover one of your eyes with this patch.

Now, can you point your arm in the direction of the Es on this line at the bottom here? I’ll point to each one, and you tell me! Ready? [Squeaks squeaks] [Squeaks squeaks] [Squeaks squeaks] [Squeaks squeaks].

Great, Squeaks! You were able to see almost all of them, so it looks like that eye is fine. Next we’ll try the other eye.

Your doctor will do this test for real at your next appointment, and she’ll keep doing it as you get older to check if you need glasses. All kids should have a vision checkup at least once a year. Kids might not talk about having problems seeing because they think that’s the way everyone sees.

It takes an eye doctor to know for sure, and many vision problems can be corrected if they’re caught early. If you live in Minnesota, learn more at GetCTC.com. And if you live somewhere else in the United States, you can click the the link in the description to learn more about the benefits that may be available to you. ♪.