There's nothing like a romantic kiss. It is the climax of every great love story and an experience that has motivated poets and musicians for thousands of years. It's also a wonderful example of a behavior that is both nature and nurture. Humans have an instinctive drive to connect through kissing, but the style and expression is shaped by our culture and personal experiences.
Beneath the surface, a kiss serves as the ultimate litmus test for a relationship. It can urge us to pursue a deeper connection with someone special or act as a warning to back off when something just doesn't feel right. Science is finally beginning to unravel the mystery of how this happens.
While we don't often think of them in this way, human lips are the body's most exposed erogenous zone. Packed with sensitive nerve endings, even a light brush sends a cascade of information to our brains helping us to decide whether we want to continue and what might happen next.
Lip contact involves five of our 12 cranial nerves as we engage all of our senses to learn more about a partner. Electric impulses bounce between the brain, lips, tongue and skin, which can lead to the feeling of being on a natural "high" because of a potent cocktail of chemical messengers involved.
A passionate kiss acts like a drug, causing us to crave the other person thanks to a neurotransmitter called dopamine. This is the same substance involved in taking illegal substances such as cocaine, which is why the novelty of a new romance can feel so addictive. Dopamine is involved in sensations of reward, making us feel intense desire that can lead to feelings of euphoria, insomnia, and loss of appetite, and it is only one actor in the great chemical ballet happening in our bodies.
And then there are physical changes. A kiss can cause our blood vessels to dilate, our pulse to quicken and cheeks to flush. Our pupils grow wide, which is likely one reason that so many of us are apt to close our eyes. In other words, the body's response mirrors many of the same symptoms frequently associated with falling in love.
But let's be honest; not every kiss leaves us wanting more.
Evolutionary psychologists at The State University of New York at Albany recently reported that 59% of men and 66% of women say that they have ended a budding relationship because of a bad kiss. How is it that such a seemingly simple exchange has the power to influence attraction dramatically?
We can readily describe sensations of butterflies in the stomach or feeling weak in the knees when things go well, but we are less aware of the ways that kissing helps us to make a reliable assessment of another person. An often-overlooked organ that's intimately involved in the experience is the nose.
Beyond obvious mood spoilers such as poor hygiene and bad breath, we each have a distinct natural scent that appears to guide us toward choosing a partner with compatible DNA. Scientists have found that women prefer the scents of men with a complementary set of genes that code for the immune system. The benefit may be that if children come along down the line, they would be well-equipped to ward off disease.
Of course, some people worry about the way that a kiss also puts us in very close proximity to another person's germs. But in reality, we're more likely to get sick by shaking hands throughout the day than by kissing.
It's necessary to include one important caveat: There seems to be a rise in the number of young people biting each other to express affection.
This may be due, in part, to vampires in Hollywood making the exchange look extremely good on the big screen. However, biting your loved ones is the least appropriate way to show you care because injecting potentially dangerous viruses and bacteria directly into someone's blood stream can be deadly.
Blood-sucking aside, kissing allows us to cross normal social boundaries and get close enough for a reliable sample of scent, taste and possibly pheromones providing insight into long-term compatibility. In other words, there's more to a kiss than meets the lips.
We may not be actively aware of all of the significant ways our bodies respond, but kisses are powerful clues to evaluate when to pursue a relationship and when to move on. Fortunately, understanding the real chemistry involved doesn't take any of the magic out of the moment, but it might boost the odds of leaving a lasting impression.
Sheril Kirshenbaum is the author of "The Science of Kissing" and a research scientist at The University of Texas at Austin. She spoke last year at the TED Global conference in Edinburgh, Scotland.
Doctor Who's 50th schedule announced
The BBC announces its schedule of programmes marking the 50th anniversary of the first-ever Doctor Who, including a 75-minute special episode called The Day of the Doctor.
'Climate change' killed off mammoths
Researchers have found the strongest evidence yet that climate change rather than humans was the main factor that drove the woolly mammoth to extinction.
Cameron outlines UN Syria resolution
David Cameron says the UK, France and the US are tabling a UN resolution on Syria designed to ensure a plan to hand over chemical weapons is "not a ruse".
Gove 'insulted' food-bank users
Opposition MPs accuse Education Secretary Michael Gove of "insulting" people who use food banks by saying they are responsible for their plight.
A&E cash linked to staff flu jabs
Hospitals in England will have to get most of their staff vaccinated against flu if they want access to the A&E bailout fund in the future.
Baby weaning foods found 'lacking'
Baby foods in the UK fail to meet infants' dietary weaning needs because they are mostly sweet foods providing little extra nutritional goodness over breast milk, researchers claim.
Apple reveals two new iPhone models
Apple unveils two iPhone at an event in California - the top-end 5S with a fingerprint sensor and a cheaper 5C which comes with a plastic back.
Shakeup for Dow Jones share index
One of the world's best known market barometers, the Dow Jones industrial average is getting a revamp with three firms set to leave.
Nigel Evans quits amid sex charges
Conservative MP Nigel Evans resigns as Commons Deputy Speaker, after being charged with offences including sexual assault, indecent assault and rape.
North's campuses 'top for nightlife'
Students seeking an active nightlife from their university days should head to the north of England or Scotland, while those wanting an active political scene may fare better in London and the south-east, a survey suggests.
NI banking practices under scrutiny
A wide-ranging inquiry into banks in Northern Ireland is due to begin at Westminster, with the future of the troubled Ulster Bank one of the key areas of investigation.