How you see
Seeing starts with light. Light bounces off everything around you, like a dog, a tree or this page. Some of that light goes into your eyes.
Your eye bends the light to make a tiny picture on the back of the eye. Then a nerve sends the picture to your brain, and your brain works out what it is: that's my dog! All of this happens faster than a blink.
Your eyeball is about 1 inch across, about as big as a gumball. Only the front part shows. The rest sits safely in a bony bowl in your skull.
The parts of your eye
The cornea (KOR-nee-uh) is a clear dome on the front of your eye, like a window. It lets light in and starts to bend it.
The iris is the colored part. It can be brown, blue, green, gray or hazel. In its middle is the pupil, the black dot. The pupil is really a hole that lets light inside.
Behind the pupil is the lens. It changes shape to focus, so things look sharp whether they are near or far.
At the very back is the retina (RET-in-uh), a thin layer full of light sensors. The optic nerve, a thick cable, carries the retina's messages to your brain.
Rods and cones
Your retina has two kinds of light sensors. Rods work in dim light and help you see shapes and movement at night. Each eye has about 120 million of them.
Cones see color and tiny details, and they work best in bright light. Each eye has about 6 million. Some cones sense red, some green and some blue, and your brain mixes their messages into every color you see.
That's why colors seem to fade in a dark room. Your cones need more light, so your rods take over, and rods only see shades of gray.
Some people's cones see certain colors, like red and green, differently. This is called color blindness. People who have it still see lots of colors, just not all in the same way.
Your brain flips the picture
Here's something strange: the picture on your retina is upside down! You can see the same trick in the bowl of a shiny spoon. Hold it at arm's length and your face looks upside down.
Your brain flips the picture the right way up, so you never notice. You have been doing this your whole life.
Your two eyes look from two slightly different spots, and your brain blends both pictures into one. That helps you tell how near or far things are.
Try it: hold up one finger, then close one eye and then the other. Watch your finger seem to jump from side to side!
Blinks, tears and pupils
You blink about 15 to 20 times a minute without thinking. Each blink spreads tears over your eye and wipes away dust, like a windshield wiper.
Your eyes make tears all the time, not just when you cry. Tears keep your eyes wet and help fight germs. Extra tears drain through tiny holes in the inner corners of your eyelids into your nose. That's why your nose runs when you cry.
Your pupil grows and shrinks to let in just the right amount of light. In bright light it gets small. In the dark it opens wide.
Try it: stand in front of a mirror in a dim room and look at your pupils. Then have a grown-up turn on the light. Watch your pupils shrink in about a second!