It would be more accurate to say that our bodies create tears for our eyes. The human eye itself is protected from outside irritants by a tear film, which is actually three layers thick. The outermost lipid layer is oily, which helps the liquid to remain in the eye. The middle aqueous layer is watery, but also contains chemicals that wash away germs and other irritants. The innermost layer of the film is called the mucous layer, and provides a smooth protective coating over the surface of the eye.
The human body produces tears by stimulating two glands situated above each eye, behind the upper eyelids. These lacrimal glands are responsible for excreting lacrimal fluid, the medical term for tears. Lacrimal fluid is very salty, although there is some evidence that the salinity level changes depending on the cause of the fluid's release. When the brain senses a need for tears, the glands excrete the lacrimal fluid into the upper eyelid area. Gravity or capillary action draws this fluid onto the eye's surface, where it either lubricates the eyeball, washes away the irritant, or simply pools at the base of the lower eyelid.
Ideally, the excess tears collect in an area known as the lacrimal lake, where they will eventually be drawn into a small opening called the lacrimal duct. If the duct becomes overwhelmed or blocked, however, the excess may spill out of the lower eye and down the person's cheek. The lacrimal duct drains into the nasalacrimal duct, where the tears may either drain into the throat or nasal passages.
The production of tears is essentially semi-automatic. The lacrimal glands routinely produce a small amount of liquid in order to keep the eyes lubricated and free of irritants. If a specific irritation, such as the gas from a raw onion or cigarette smoke, reaches the eyes, then the brain can send out a signal for the lacrimal glands to produce more.
A person can also generate tears through the use of strong emotions. Trained actors often have the ability to generate them in a dramatic scene by using sense memory to trigger a genuine emotional response. So-called "crocodile tears" can also be generated voluntarily by channeling a false emotional response for the benefit of others. Hard squinting can also force lacrimal fluid to pool up, creating the illusion of real tears.