Why Does Skin Get Pruney in the Tub?

For a long time, scientists thought that pruning of the skin after spending time in the water was simply a matter of fingers being a little spongey. The outermost layer (the stratum corneum) of the outermost layer (the epidermis) of our skin is mostly made up of cells called corneocytes. These cells are filled with keratin, a protein that helps keep the skin hydrated by absorbing water and preventing its evaporation. When you hang out in the pool or the bathtub for a while, the keratin absorbs a lot of water, and the cells swell up. While the thin stratum corneum swells with water, the lower layers of skin that it is attached to don’t, so that outermost layer has to buckle and bend to accommodate its relatively larger size, sort of like a too-big shirt that wrinkles and bunches together when it's tucked in.

Another, more recent explanation is that the wrinkles come from vasoconstriction, or the narrowing of blood vessels. The idea is that hot water makes the blood vessels in the fingers tighten and the surrounding tissue contract, causing the skin to fold.

But the explanation might be more complicated than either one of those potential causes—especially when you consider how the phenomenon occurs in people with nerve damage.

So Unnerving

In the 1930s, two scientists examined a boy whose median nerve was severed, leaving his thumb, index, and middle fingers numb. When they soaked his hand in water, the ring and pinkie fingers wrinkled but the fingers affected by the damaged nerve stayed smooth.

And in 2001, researchers at Tel Aviv University found that nervous system malfunctions caused by Parkinson’s disease also interfered with finger wrinkling. In their study, Parkinson’s patients’ fingers wrinkled less on one side of the body than the other, and wrinkled less overall than the fingers of healthy subjects. Going by the common explanations, the wrinkles were a local phenomenon happening in very small bits of flesh. The involvement of the nervous system, though, suggests that something else is going on.

Getting a Grip

Mark Changizi, neuroscientist and the Director of Human Cognition at 2AI Labs in Boise, Idaho, thinks that the wrinkles’ neural factor is a clue that they’re adaptive. Rather than being a mere side effect of water-logged digits, he says, they’re a functional response to wet conditions: The wrinkles act like drainage networks or tire treads on our fingers and toes, channeling water away and giving skin more contact with, and a better grip on, wet surfaces.

Analyzing the wrinkles on various soaked fingers, Changizi and his team found that they all had similar shapes and characteristics—with disconnected channels that moved away from each other as they got farther from the fingertip—consistent with what is expected in a drainage network. That wasn’t much evidence for Changizi’s hypothesis, but it got the ball rolling. (Update: 11/30/2012, 1:25 pm) While that doesn't seem like much, Changizi points out that the "morphology prediction is actually very strong."

"Of the infinitely many wrinkle patterns that are possible," he says, "[the] drainage hypothesis predicts [the] actual [pattern]."

Since publishing the idea and the initial data last year, he and his team have been looking for evidence of finger-wrinkling in other primates that live in wet environments (they’d already found it happens in Japanese macaques) and are setting up experiments to directly test the wrinkles' effects on grips, While the results aren't ready to be published yet and the pilot studies so far suggest that pruney fingers do help improve grip.

(Update: 11/30/2012, 1:25 pm) Changizi has filled me in on that pilot data. The experiment was conducted Changizi and undergraduate student Joseph Palazzo. They had subjects carry out a timed task of moving objects, including bottles, stones, logs and other items, from one place on a table top to another, and back again. They did this in wet-pruney, dry-pruney (dry objects, and fingers dried after they had wrinkled), wet-nonpruney (wet fingers, but not yet wrinkled) and dry-nonpruney conditions. Wet-pruney performance was better than wet-nonpruney, with the subjects being faster and making fewer mistakes.

Changizi would like to see more behavioral studies like this carried out, and see more data from other species for further tests, but  probably won’t carry out any of these studies himself. "A more sophisticated next experiment would be version-2.0 of this sort of thing, in my mind," he says. "But not my forte." He thinks that other scientists would be much better at that kind of experiment.

"In terms of the categories of test, then," he says. "There's morphology, behavior, and phylogeny, and at this point we've done the first, poked at the second, and only wondered about the third."

Big Questions
Why Do Onions Make You Cry?

The onion has been traced back as far as the Bronze Age and was worshipped by the Ancient Egyptians (and eaten by the Israelites during their bondage in Egypt). Onions were rubbed over the muscles of Roman gladiators, used to pay rent in the Middle Ages, and eventually brought to the Americas, where today we fry, caramelize, pickle, grill, and generally enjoy them.

Many of us burst into tears when we cut into one, too. It's the price we pay for onion-y goodness. Here's a play-by-play breakdown of how we go from grabbing a knife to crying like a baby:

1. When you cut into an onion, its ruptured cells release all sorts of goodies, like allinase enzymes and amino acid sulfoxides. The former breaks the latter down into sulfenic acids.

2. The sulfenic acids, unstable bunch that they are, spontaneously rearrange into thiosulfinates, which produce a pungent odor and at one time got the blame for our tears. The acids are also converted by the LF-synthase enzyme into a gas called syn-propanethial-S-oxide, also known as the lachrymatory factor (or the crying factor).

3. Syn-propanethial-S-oxide moves through the air and reaches our eyes. The first part of the eye it meets, the cornea, is populated by autonomic motor fibers that lead to the lachrymal glands. When syn-propanethial-S-oxide is detected, all the fibers in the cornea start firing and tell the lachrymal glands to wash the irritant away.

4. Our eyes automatically start blinking and producing tears, which flushes the irritant away. Of course, our reaction to burning eyes is often to rub them, which only makes things worse since our hands also have some syn-propanethial-S-oxide on them.

It only takes about 30 seconds to start crying after you make the first cut; that's the time needed for syn-propanethial-S-oxide formation to peak.


The onion's relatives, like green onions, shallots, leeks and garlic, also produce sulfenic acids when cut, but they generally have fewer (or no) LF-synthase enzymes and don't produce syn-propanethial-S-oxide.


Since I usually go through a good deal of onions while cooking at home, I've been road testing some of the different methods the internet suggests for reducing or avoiding the effects of the lachrymatory factor. Here's what I tried:

Method #1: Chill or slightly freeze the onions before cutting, the idea being that this will change the chemical reactions and reduce the gas that is released.
Result: The onion from the fridge has me crying just as quickly as room temperature ones. The one that was in a freezer for 30 minutes leaves me dry-eyed for a bit, but by the time I'm done dicing my eyes start to burn a little.

Method #2: Cut fast! Get the chopping over with before the gas reaches your eyes.
Result: Just hacking away at the onion, I get in the frying pan without so much as a sting in my eyes. The onion looks awful, though. Doing a proper dice, I take a little too long and start tearing up. If you don't mind a mangled onion, this is the way to go.

Method #3: Put a slice of bread in your mouth, and cut the onion with most of the bread sticking out to "catch" the fumes.
Result: It seems the loaf of bread I have has gone stale. I stop the experiment and put bread on my shopping list.

Method #4: Chew gum while chopping. It keeps you breathing through your mouth, which keeps the fumes away from your eyes.
Result: This seems to work pretty well as long as you hold your head in the right position. Leaning toward the cutting board or looking right down at the onion puts your eyes right in the line of fire again.

Method #5: Cut the onions under running water. This prevents the gas from traveling up into the eyes.
Result: An onion in the sink is a hard onion to cut. I think Confucius said that. My leaky Brita filter is spraying me in the face and I'm terrified I'm going to cut myself, but I'm certainly not crying.

Method #6: Wear goggles.
Result: In an effort to maintain my dignity, I try my eyeglasses and sunglasses first. Neither do me any good. The ol' chemistry lab safety glasses make me look silly, but help a little more. I imagine swim goggles would really do the trick, but I don't have any.

Method #7: Change your onion. "Tear free" onions have been developed in the UK via special breeding and in New Zealand via "gene silencing" techniques.
Result: My nearest grocery store, Whole Foods, doesn't sell genetically modified produce or onions from England. Tonight, we eat leeks!

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Big Questions
What is Mercury in Retrograde, and Why Do We Blame Things On It?

Crashed computers, missed flights, tensions in your workplace—a person who subscribes to astrology would tell you to expect all this chaos and more when Mercury starts retrograding for the first time this year on Friday, March 23. But according to an astronomer, this common celestial phenomenon is no reason to stay cooped up at home for weeks at a time.

"We don't know of any physical mechanism that would cause things like power outages or personality changes in people," Dr. Mark Hammergren, an astronomer at Chicago's Adler Planetarium, tells Mental Floss. So if Mercury doesn’t throw business dealings and relationships out of whack when it appears to change direction in the sky, why are so many people convinced that it does?


Mercury retrograde—as it's technically called—was being written about in astrology circles as far back as the mid-18th century. The event was noted in British agricultural almanacs of the time, which farmers would read to sync their planting schedules to the patterns of the stars. During the spiritualism craze of the Victorian era, interest in astrology boomed, with many believing that the stars affected the Earth in a variety of (often inconvenient) ways. Late 19th-century publications like The Astrologer’s Magazine and The Science of the Stars connected Mercury retrograde with heavy rainfall. Characterizations of the happening as an "ill omen" also appeared in a handful of articles during that period, but its association with outright disaster wasn’t as prevalent then as it is today.

While other spiritualist hobbies like séances and crystal gazing gradually faded, astrology grew even more popular. By the 1970s, horoscopes were a newspaper mainstay and Mercury retrograde was a recurring player. Because the Roman god Mercury was said to govern travel, commerce, financial wealth, and communication, in astrological circles, Mercury the planet became linked to those matters as well.

"Don’t start anything when Mercury is retrograde," an April 1979 issue of The Baltimore Sun instructed its readers. "A large communications organization notes that magnetic storms, disrupting messages, are prolonged when Mercury appears to be going backwards. Mercury, of course, is the planet associated with communication." The power attributed to the event has become so overblown that today it's blamed for everything from digestive problems to broken washing machines.


Though hysteria around Mercury retrograde is stronger than ever, there's still zero evidence that it's something we should worry about. Even the flimsiest explanations, like the idea that the gravitational pull from Mercury influences the water in our bodies in the same way that the moon controls the tides, are easily deflated by science. "A car 20 feet away from you will exert a stronger pull of gravity than the planet Mercury does," Dr. Hammergren says.

To understand how little Mercury retrograde impacts life on Earth, it helps to learn the physical process behind the phenomenon. When the planet nearest to the Sun is retrograde, it appears to move "backwards" (east to west rather than west to east) across the sky. This apparent reversal in Mercury's orbit is actually just an illusion to the people viewing it from Earth. Picture Mercury and Earth circling the Sun like cars on a racetrack. A year on Mercury is shorter than a year on Earth (88 Earth days compared to 365), which means Mercury experiences four years in the time it takes us to finish one solar loop.

When the planets are next to one another on the same side of the Sun, Mercury looks like it's moving east to those of us on Earth. But when Mercury overtakes Earth and continues its orbit, its straight trajectory seems to change course. According to Dr. Hammergren, it's just a trick of perspective. "Same thing if you were passing a car on a highway, maybe going a little bit faster than they are," he says. "They're not really going backwards, they just appear to be going backwards relative to your motion."

Embedded from GIFY

Earth's orbit isn't identical to that of any other planet in the solar system, which means that all the planets appear to move backwards at varying points in time. Planets farther from the Sun than Earth have even more noticeable retrograde patterns because they're visible at night. But thanks to astrology, it's Mercury's retrograde motion that incites dread every few months.

Dr. Hammergren blames the superstition attached to Mercury, and astrology as a whole, on confirmation bias: "[Believers] will say, 'Aha! See, there's a shake-up in my workplace because Mercury's retrograde.'" He urges people to review the past year and see if the periods of their lives when Mercury was retrograde were especially catastrophic. They'll likely find that misinterpreted messages and technical problems are fairly common throughout the year. But as Dr. Hammergren says, when things go wrong and Mercury isn't retrograde, "we don't get that hashtag. It's called Monday."

This story originally ran in 2017.


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