A clear, blue sky, stars twinkling at night, the faces of people you love--you enjoy all of these because of the king of all the senses, vision. Two thirds of what you know about the world you learn through your eyes and one third of the thinking part of your brain is devoted to processing light so that you can make sense of it.

Seeing begins when light passes through the cornea (the curved, clear dome in the front of your eye) and continues through the aqueous humor, a fluid behind the cornea. The light next travels through the pupil, the hole made by your iris. The iris can contract or relax to control the amount of light that reaches the lens.

The lens is like a flexible magnifying glass; muscles change its shape to focus the light through the vitreous humor (jelly that fills your inner eyeball) and onto the retina. Like film in a camera, the retina is sensitive to light. Its millions of cells send electrical impulses to vision centers in the brain when struck by light. The retina is actually part of the brain itself that has extended into your eyeballs.

Q. Are two eyes better than one?

A. Some animals, like rabbits, have their eyes on the sides of their heads, letting them see to either side and even behind themselves. That's because they spend most of their waking hours browsing for food; being able to see in all directions guards against other animals trying to sneak up on them.

Human beings evolved as predators and have forward-facing eyes. The slight distance between them helps our brains figure out how far away something is (an ability called depth perception), which is handy when you're trying to catch your dinner. This game shows how depth perception is lost when you use only one eye:

MATERIALS

• Wastebasket
• Three small balls
• Blindfold

Stand five feet away from a wastebasket and try tossing the balls into it. Next, tie a blindfold so it covers one eye and try sinking the balls again. Since your brain needs two eyes to compute depth, you'll probably have more trouble making your shots when using just one eye.

Q. What are the specks I sometimes see when I look at a bright light?

A. The specks are harmless dead cells or cell fragments floating around in the fluid inside your eye.

Q. Why do I sometimes have to turn my head to see something clearly?

A. You do this to help your eyes focus light on the fovea, a spot on the retina that has the greatest concentration of cones. It's in the fovea that vision is sharpest. People who have glaucoma slowly become blind as extra fluid pressure inside their eyes kills the cells in their retinas. Since the fovea has such a high concentration of cells, vision there lasts the longest. Some people with glaucoma have 20/20 vision, but their field of view is so narrow they see as if they were looking through a straw.

FACTOIDS

• People see colors differently--what one person calls red may look to you like pink.
• Optical nerves carry thirty times as many nerve fibers as auditory nerves.
• Human eyes have large blind spots. We don't notice them because our brains fill in the missing information.
• Very few people are born without eyesight. Most people lose their vision because of disease.