Short-sightedness (myopia) usually occurs when the eyes grow slightly too long, which means they're unable to produce a clear image of objects in the distance.
It's not clear exactly why this happens, but it's thought to be the result of a combination of genetic and environmental factors that disrupt the normal development of the eye.
Light passes through the cornea (the transparent layer at the front of the eye) and into the lens (the transparent structure that sits behind the cornea).
This focuses it on to the retina (a layer of light-sensitive tissue at the back of the eye) to create an image that's then sent to the brain.
To produce a perfectly clear image, the cornea should be evenly curved and the eye needs to be the right length.
In people with short-sightedness, the eye has usually grown slightly too long.
This means that when you look at distant objects, the light isn't focused directly on to your retina, but a short distance in front of it.
This results in a blurry image being sent to your brain.
Although it's not clear exactly why some people become short-sighted, there are some things that can increase your chances of developing the condition.
Short-sightedness is known to run in families, so you're more likely to develop it if one or both or your parents are also short-sighted.
Research has so far identified more than 40 genes linked to short-sightedness.
These are responsible for the eye's structure and development, and signalling between the brain and the eyes.
Too little time outdoors
Research has found that spending time playing outside as a child may reduce your chances of becoming short-sighted, and existing short-sightedness may progress less quickly.
This may be related to light levels outdoors being much brighter than indoors.
Both sport and relaxation outdoors appear to be beneficial in reducing the risk of short-sightedness.
Excessive close work
Spending a lot of time focusing your eyes on nearby objects, such as reading, writing and possibly using hand-held devices (phones and tablets) and computers can also increase your risk of developing short-sightedness.
An "everything in moderation" approach is therefore generally recommended.
Although children should be encouraged to read, they should also spend some time away from reading and computer games each day doing outdoor activities.