Visit any playground on a warm summer day, and you'll see them: kids whirling on merry-go-rounds, playing ring-around-the-rosie and simply spinning around and around until they fall down.

Try to join them as an adult, however, and you're more likely to turn green around the gills than to match their gleeful giggles.

Generally speaking, kids love getting dizzy; adults, less so. There are exceptions, of course (whirling dervishes, circus performers), but for the most part, our desire to spin until the world goes topsy-turvy seems to peter out after childhood. Nausea replaces joy.

And oddly enough, no one is quite sure why this is.

"It's a very complicated question," said Devin McCaslin, an audiologist at Vanderbilt University Medical Center.

"You're basically asking a developmental and a nausea question and both of those are not well understood," said Jorge Serrador, a scientist at Rutgers New Jersey Medical School who studies the vestibular, or balance, system.

Wait, really?

Really, Serrador says. "There haven't been a lot of studies looking at vestibular development throughout age."

Okay. So there isn't a scientific slam-dunk answer to this question. But researchers do have some theories. The most compelling, they say, is that spinning around isn't just fun for kids. It actually trains their balance system, which is under construction until about age 15.

Wired for balance

At 36 weeks development in the womb, the balance sensors in the inner ear are completely formed, said McCaslin. The vestibular system consists of three semicircular ducts, which are a small tangle of fluid-filled tubes. As the head moves, the fluid moves too, bending tiny hair-like cells that relay messages to the brain about head position. The semicircular ducts are largely responsible for detecting information about rotational movement, Serrador said.

Meanwhile, teeny-tiny stones called otoliths sit in a pair of little sacks called the saccule and utricle, both of which are tucked between the semicircular ducts and the cochlea, the snail-shaped organ that turns sound waves into nerve impulses. The otoliths act as little rolling weights that bend hair-like receptor cells as they move about in response to head motion.

Meanwhile, the visual system adds its two cents, and receptors in the joints convey information about the body's balance, too. To get a sense of how all three of these systems work together, stand on one foot. Once you've got your balance, close your eyes. That wobble you felt was your brain compensating for the sudden loss of one of your balance information streams.

As balance is a pretty crucial task for a mobile organism, the job of processing all of this information goes to two evolutionarily old brain areas — the brainstem and the cerebellum.

It's the connections to and within the cerebellum that aren't fully developed until adolescence, McCaslin said. As kids proceed through their developmental milestones — lifting the head, rolling over, crawling, walking and running — this system is wiring and rewiring. The input that comes from spinning around for fun may be a part of this process.

"The system needs that," McCaslin said. "It really, really needs to be stimulated."

From whirling to hurling

The fact that kids love and need dizziness, however, doesn't explain why spinning around becomes actively unpleasant for many adults.

Some kids do hate dizziness, particularly those who get dizzy as a symptom of migraines, McCaslin said. And some kids are more susceptible to nausea than others, just like adults.

No one really knows why dizziness causes nausea, or why some people are more susceptible than others. The prevailing theory, Serrador said, is something called "intravestibular conflict." If you're below decks on a pitching and yawing boat, your inner ear will detect the motion of the waves. To the eye, however, the world looks stable. Likewise, tilting and whirling around at certain frequencies confuses the vestibular system. So does sitting in a moving car while looking at a stationary object, like a book or smartphone.

Why does this confusion translate to nausea? One theory, McCaslin said, is that the brain decides that all of this conflicting information must mean that you ate something toxic. In response, it tries to get you to throw up.

Serrador and his colleagues have found that about 5 minutes before people report nausea after spinning, the blood flow to their brain goes down. It's an interesting finding, he said, but it's not yet clear if or why the decreased blood flow translates to queasiness.

Your aging inner ear

The vestibular system becomes less efficient with age, which is one of the reasons that elderly people often have balance problems. The otoliths, in particular, degenerate. But because otoliths return more information about linear acceleration than rotation, it seems unlikely that this degradation is the cause of adults' trouble on the merry-go-round, Serrador said. It's possible, though, that otolith degradation causes a change in the ratio of information the brain gets from the otoliths and semicircular canals, he said. That might cause vestibular confusion.

Viruses and infections can cause inner ear damage, too. Kids, with their malleable brains, compensate with startling ease.

"There are kids without vestibular systems who get through life and they didn't even know until we tested them," McCaslin said.

Adults who lose both inner ears are in trouble, McCaslin said. They lose the visual reflex that keeps the eyes steady as the head moves, so the world looks like a poorly-filmed clip from a handheld video camera. But for people with simple dizziness or vertigo, simple exercises like turning the head back and forth can retrain the balance system, according to research by Lucy Yardley, a health psychologist at the University of Southampton. If an adult loses function of one inner ear, he or she can recover balance in a matter of months as the brain learns to compensate, McCaslin said.

And most people can be trained to tolerate dizziness. Air Force pilots, Navy flyers and astronaut candidates all undergo spinning exercises to tamp down their nausea response. This desensitization process trains the brain to suppress information coming from the inner ear in favor of visual cues, Serrador said. This is a temporary fix, however; not long after a person stops training, their tolerance for spinning will drop again.

There might be some aspect of "use it or lose it" in the decrease of tolerance for dizziness with age. "Older people's balance systems would benefit from [spinning] too," said Southamptom's Yardley.

All the more reason to hop on that merry-go-round and take it for a spin — but maybe start slow.