More and more people need to wear glasses, including young children, and we really don't know why. Our eyes allow us to see because light enters each eye, and the eye then produces a message that is sent to the brain.
The eyeball itself doesn't actually "see", it's the brain that sees it. The eyeballs just take pictures, like two little cameras. To see properly, each eyeball needs to send light that enters it to a very precise location within the eyeball, called the retina. If the light falls in the wrong place, your vision will be blurred.
Many people can see clearly without glasses. This is because their eyeballs focus light correctly onto the retina. However, some people's eyeballs are too long. They are called "short-sighted". For these people, distant objects, such as street signs or classroom blackboards, may appear blurry.
Others have eyeballs that are too short. They're called "visionaries," and things near them, like books or cell phones, may appear blurry. Both nearsighted and farsighted people need glasses to help them see clearly. They work by helping the eyeball focus light into the right place, the retina. Only then can the eyes see clearly.