Lip skin is so thin. This makes lips very sensitive to air, fluids, and solids. The best way to feel, to understand the situation of a rock, for example, is to place it on your lips. Its hardness comes to life. Other parts of your skin, like the back of your hand, diminish this hardness, and the rock's rockiness recedes. It is for this reason, the superior sensitivity, that we kiss things like the "ouchie" on a baby, or the cover of a beloved book—because kissing expresses sensitivity by placing the cherished person or thing on a sensitive part of our body.

That's one type of kissing, and it's practiced by 90 percent of humans and many animals—simply Google "kissing animals" and you will see all sorts of animals being friendly with the lips (squirrels, hippos, chimps, chipmunks, rabbits, tigers, and also a tiger with a monkey, a deer with a fox, a dolphin with a dog). The other type of kissing is sexual, and only three great apes—gorillas, bonobos, humans—are known to cross the lip line between friendship and lover. Sexual kissing is all about open mouths. And it is here that a moment of risk presents itself. When you kiss a person, open your mouth to the other mouth, not only are saliva, the remains of a meal, the residue of a glass of wine exchanged, all sorts of nasty invisible other things are exchanged, too. "As saliva exchange, even between healthy individuals, involves exposure to organisms that can cause dental caries, periodontitis, gingivitis, oral candidiasis, etc., it clearly carries a significant risk," write C. A. Hendrie and G. Brewer in the paper "Kissing as an Evolutionary Adaptation to Protect Against Human Cytomegalovirus-Like Teratogenesis."

But why in the world would a person in their right mind open her/himself to such a risk? Why do all of us so willingly do it? Indeed, if your move to sexually kiss someone is rejected, how fast and hard the spirits sink. We have all been there: The night is totally ruined, you drink to oblivion, you wake up with a pain in the head and the heart. Why does it mean so much to make out? What function does it serve?

Let us enter a jungle in Costa Rica and observe some monkeys doing something strange. The monkeys are white-faced capuchins; they are very social, have big brains for their small size, live in tropical dry forests, and occasionally eat baby squirrels and iguanas. As these little creatures move about the trees, they make terrible noises and shake leaves horribly. Then it happens: One capuchin monkey sticks its finger into another capuchin's eyeball. The finger goes deep into the socket, deep between eyelid and eye. The ball is almost popped out. The monkey with the finger in its eye makes no sudden moves. The fingernail in its socket is long and filthy. One wrong move and the fingernail could cut the eye and cause blindness or an infection that could kill it good. The capuchin is frozen as the finger of its friend goes deeper and deeper.

This monkey business is called "eyeball poking" and is considered to be one of several "socially transmitted behaviors" in capuchin culture. The monkeys also engage in finger sniffing, which is very common and involves one monkey giving the other a finger to sniff (the practice often leads to nostril poking). There is the sucking of a body part, which involves one monkey inserting an ear or tail into another monkey's mouth (the monkey will suck the body part for what seems to be a very long time). And there is the biting of backs, which involves one monkey tearing hair off the back of another monkey and then offering the tuft of hair to the bitten and back-sore monkey (the hair moves from one mouth to another).

All of these bizarre behaviors—the most dangerous of which is, of course, eyeball poking—are close cousins of the behavior of human sexual kissing. What relates the two is the element of risk—the monkey risks losing an eye or dying, the human risks catching some disease that could leave them in bed for a week.

And why take the risk at all?

Because it is an expression of trust.

I learned about the white-faced capuchin monkeys from a lecture by the American anthropologist Susan Perry, who got her ideas about why monkeys engage in this risky behavior from the Israeli evolutionary biologist Amotz Zahavi. There are beautiful poems, beautiful essays, and beautiful novels—these are common enough in the world of writing. What's rare are beautiful academic papers. One such paper is Zahavi's "The Testing of a Bond," published in 1977.

"An assessment of the strength of a bond may be arrived at through observation per se," writes Zahavi. "But it may be easier and quicker to discriminate between an individual which is likely to form a bond and another which is not by imposing a test which will be accepted by the one and rejected by the other. An easy way to perform such a test is to inflict a stress on the tested individuals. The stress will be a condition for the formation of maintenance of the bond. Under these circumstances an individual which is not interested in the bond may not be ready to sustain the stress, while one which intends to maintain the bond will do so. It is also reasonable to assume that the reaction to the test should be a measure of the readiness of the individual to bond."

What this means is that cultural practices such as kissing and eyeball poking flourish in the social environments of intelligent animals because they make visible the invisible—a bond between two beings. An asocial animal does not need to test anything with anybody. It knows nothing of this urge to externalize a desire, a feeling that is inside. The world begins and ends with what is in their heads. When a highly social animal opens their lips during a kiss, they are signaling a feeling of trust that is within them. And if the other welcomes this opening with an opening of their own—this signals their trust for your trust. A connection is made. Pleasure is exchanged.

If, however, your mouth opens and the other's remains sealed, this means they do not want to trust you. The willingness to take the risk flows only one way; it flows into a void. Your mouth lacks the allure of a risk. It is to the one you desire nothing but a pit of bugs and spit. What they are willing to offer you is a sensitive kiss on the cheek (at best) or a hug (at worst). Both of these socially transmitted practices are safe.

One of the most powerful horror images in the popular imagination is a vagina with teeth. This monster recently appeared in the movie Teeth. A young woman bites off not only penises but also the fingers of men who are fingering her. Enter her and, at the highest moment of sexual joy, the moment before ejaculation, you are cut. Lots of screaming and blood mixed with sperm.

Contrast this "monster" with a regular mouth. If a toothed vagina is so scary, why is there no fear of a mouth, a lover's mouth, that's open to suck and pleasure a penis? It is exactly the same thing as a toothed vagina, but we do not see it as something terrifying. In fact, it is something many men beg for—the sensation of a mouth—even though human mouths are filled with teeth. Why do we not fear placing a fleshy and blood-packed member into a cavity that contains the hardest parts of the human body, those molars and incisors, those grinders and cutters, those chewers and rippers?

Because a blowjob is all about trust.

A blowjob, like kissing, is a great way to test a bond. To bite and inflict pain is a great way to break that bond and bring a relationship to an end. True, there is always an element of pain and even aggression in most sexual tests—the playful bite on the lip (eating), the squeezing of a wrist (seizing), the pressing against the body (forcing). Indeed, Zahavi believes that a certain amount of aggression is found in every act of sexual love. "Among well-established bond members," writes Zahavi, beautifully, "activities which signal love, like... allogrooming and allopreening, kissing, etc., are often still mixed with genuine aggression as manifested by a reaction which often signals unpleasant feelings from the passive participant. A bout of allopreening among Babblers often terminates when the preened bird moves its head away as if it was hurt. Embraces and the leaning of one member of a couple on another are often terminated when one cannot stand anymore the physical stress involved. These stresses are very appropriate if such an activity is meant to test the bond."

No man in his right mind would ever place his penis into the mouth of a person he does not trust. (This is why porn films about men putting their penises into the mouths of sleeping women are so terrifying—if she wakes up, her teeth will cut, bite... blood.) The trusted person in their right mind would never do more harm to the mouthed penis than press or tease it with their teeth—that's as far as their aggression will go. So if there were teeth in a vagina (even sharp, sharklike teeth), a man would not enter it if he did not trust the woman. Only a rapist could fear vagina dentata. And only a small number of men are actual sex monsters. Most men, and most women, are extremely social beings; they want to share the joy and the risks with their significant other.

Afruit "dissolves into tastes," writes Paul ValĂ©ry in Charmes. But the tongue can only taste things that dissolve on it—salty, sour, sweet. Something that is solid has no taste. When in the depths of a sexual kiss, each person tastes and relishes the raw skin of the other. It's not like eating meat or sugar; that's another kind of love—the love of eating food. This is a spiritualized eating—something that can only be described enigmatically as the love of eating love.

The pleasures of consumption, which involves the destruction of another animal or plant into smaller and smaller bits that are then swallowed and digested—kissing is this without the destruction, consumption, and assimilation of something that was once an animal doing its own thing. Real eating is a one-sided pleasure; for one side, it is a good encounter, for the cow or egg or nut, it is not so good. Kissing is eating as production, as creation.

A bad kisser is either (1) a person who actually eats you or (2) a person who does it all wrong. The second type of bad kisser puts too much of their teeth into the moment, or their tongue behaves like a panicked lizard, or their mouth can never strike that wonderful balance between rough and soothing. A bad kisser often means the deal is over. We disengage because we see them as socially inferior—they remove the magic from the risk. The bad kisser reveals their soul: They are a bad person. A good kisser is always a good person. A kiss that lasts for five minutes burns 10 calories. recommended

This article has been updated since its original publication.