Tears are a lot more complicated than they seem.
They are the body's natural mechanism for protecting the surface of the eye from infection and from the corrosive effects of dirt, dust, and other airborne particles. They help to create a smooth corneal surface so vision stays clear and undistorted, and your eyes stay comfortable. Tears provide a wet, lubricated surface that sits atop the corneal epithelium (surface of the eye).
There are three layers to the tear film. The oily, outer layer prevents evaporation. The middle layer is more watery. The inner layer contains mucous, and forms the bond between the tear film and the corneal epithelium.
Tears are constantly being replenished, so adequate tear production is important to maintain the health, comfort, and infection-fighting capabilities of the eye. When the eye can't produce enough tears to perform these functions, artificial tears or occlusion (plugging of the tear ducts) may be necessary.