Where does a tear come from? How is it formed? Also, what triggers a tear? Is it the emotions, the soul, or spirit? Is it limited to humans?

Tears come from two places. First, from tiny glands (called the lachrymal glands), about the size and shape of a shelled almond, located just above the outer corner of each eye. Also, cells in eye membranes make tears-- the ones that line the eyelids and cover the exposed part of the eyeball.

These glands and cells constantly form tears. The tears ooze through tiny ducts from the lachrymal glands into the eyelid membrane and thence over eyeball membranes where they flush the eye. That happens all the time, automatically, to prevent eyes from drying, which can harm vision and even lead to infection. Events (like a fleck of dust landing on the cornea, bright lights shining into the eye, or eating chilli peppers) can trigger a tear reflex response. Strong emotions--both laughter and sorrow-- can also cause tears to flow.

The brain triggers emotional releases, such as laughing and crying. Perhaps the source of such feelings is indeed the soul or spirit but that's beyond the realm of science.

Animals do shed tears, however. A crocodile's third eyelid flushes tears across the eyeball to lubricate its passage and to clean and protect the eye. Birds moisten their eyeballs with tears while flying. An aardvark's tear membrane protects its eyes against termite bites. Polar bears, houses, and dogs produce tears too.

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