Let us count the ways.
Tears well up in people's eyes for a slew of reasons. A teardrop running down the cheek is the ultimate symbol of sadness, but people may also cry because they just cut an onion, or maybe they're trying to blink out a rogue eyelash.
For the most part, tears help maintain healthy eyes, experts told Live Science. For instance, basal tears flow continuously from the lacrimal glands, or the tiny almond-shaped structure in the inside corner of the eye. When you blink, basal tears clean and lubricate the eyes. This makes the eyes' outer surface smooth, and helps you to see clearly, according to the American Academy of Ophthalmology.
Moreover, reflex tears, or those that happen unconsciously, help clear the eyes of irritants, including specks of dust and the sulfur-based gas that's produced when a knife slices a raw onion, said André Silva, a doctoral student of psychology at the University of Minho in Portugal. [Why Does Slicing Onions Make Me Cry?]
Emotions can also trigger tears in humans. These emotional tears (also called psychic tears) can serve as a means of nonverbal communication, Silva said.
"Babies cry to signal to their mothers [that] something's up," Silva told Live Science in an email. "It's a pure attention call."
Some people think that babies are capable of manipulating adults by crying, but that idea is false, Silva said. "This is misleading, as a baby is not conscious of being manipulative: In a way, we can consider these tears as being somewhere between basal/reflex tears and emotional tears."
As people grow older, they begin to use their tears to express emotion, such as happiness or sadness. These tears may promote compassion and empathy in others, Silva said.
But beware of people who may use their tears to manipulate others, he said. "Some sensitive people may be easily manipulated through the crying of others, and may easily shed tears that can also be used by others to manipulate them," he said.
So, emotional tears have both advantages and disadvantages. "It's a powerful communication device and a powerful manipulative device," Silva said. Crying also makes it hard for some people to mask their feelings when they're in the middle of an extreme emotional state, he said.
Basal, reflex and emotional tears each have their own chemistry, studies show.
Basal tears have three layers: a thin mucus layer that sits directly on the eye, a watery layer in the middle and a thin oily layer on top that prevents the tear from evaporating, according to Ted-Ed, a site that shares free educational videos. Reflex tears also have these three layers, but these tears have a larger watery portion and higher levels of antibodies that can stop harmful microorganisms as compared to basal tears, TedEd reported.
Emotional tears can contain higher levels of stress hormones, including leucine enkephalin, an endorphin and natural painkiller, TedEd said. This may explain the cathartic effect of "a good cry" — which is still debated — in case you need to shed a few tears.
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Laura is the archaeology and Life's Little Mysteries editor at Live Science. She also reports on general science, including paleontology. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.