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Why Do We Bite Our Nails?

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It can happen anywhere. Maybe you're waiting in the lobby of your doctor's office or studying at home for a big test—and then, without you even registering it, your fingertips are in your mouth and your teeth are gnawing away.

Even the people who do it tend to see nail biting as a nasty habit, but that doesn't make it any less prevalent. Twenty to 30 percent of people admit to biting their nails on a regular basis. Though reasons for the behavior vary, research has shown that most people do it because they need a psychological crutch, and not because they like having dirty, ragged fingernails.

According to Kieron O'Connor, a researcher at the University Institute of Mental Health at Montreal, there’s no one mood associated with nail biting. “People think you only bite when you’re stressed, but it’s not so simple,” O'Connor tells Mental Floss. “People will also bite when they’re bored and alone.” Nail biting feels good (in moderation at least), so when some people have nothing to do, or want to avoid the task in front of them, they bring their nails to their mouth out of habit. This same thing happens when nail biters feel overwhelmed by a stressful situation, or when they’re ruminating over some past behavior they feel embarrassed about. In all cases, nail biting is used as a mood regulator: a tool that distracts them or provides temporary relief or stimulation whenever an unwanted feeling comes up.

A study co-authored by O’Connor found that nail biting is more common in perfectionists. After speaking with 48 subjects, half with habit disorders like nail biting and half without, they found that the nail biters tended to be organizational perfectionists, or people who tend to over-plan, work too much, and become restless when they don’t have enough to do. “When people are perfectionists, they get bored and frustrated very easily and hold themselves to higher standards,” O’Connor says. “They plan too much, and when they can’t do all of it they have a sense of failure.”

So what is it about nail biting specifically that makes some people turn to the behavior when they’re feeling less than fantastic? Like many of our worst habits, nail biting is an evolutionary adaptation kicked into overdrive. Even before we had access to aisles worth of beauty products, humans practiced self-grooming, which includes getting rid of the occasional hangnails. But even nail biters with short, clean nails will engage in the behavior: They use it as a way to get the mental reward our brains associate with grooming without any of the cosmetic benefits.

Nail biting manifests itself at different levels of intensity. At one extreme there’s onychophagia, a condition where compulsive nail biting requires medical attention. Like skin picking and hair pulling, nail biting can be a symptom of a body-focused habit disorder. When it gets to the point that someone is harming themselves by chewing their nails so often, they usually require a mental health professional to help them curtail the behavior.

But for far more people, nail biting is just an annoying habit that’s hard for them to quit—no matter how much the squeamish people around them wish they would.

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What Happens When You Flush an Airplane Toilet?
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For millions of people, summer means an opportunity to hop on a plane and experience new and exciting sights, cultures, and food. It also means getting packed into a giant commercial aircraft and then wondering if you can make it to your next layover without submitting to the anxiety of using the onboard bathroom.

Roughly the size of an apartment pantry, these narrow facilities barely accommodate your outstretched knees; turbulence can make expelling waste a harrowing nightmare. Once you’ve successfully managed to complete the task and flush, what happens next?

Unlike our home toilets, planes can’t rely on water tanks to create passive suction to draw waste from the bowl. In addition to the expense of hauling hundreds of gallons of water, it’s impractical to leave standing water in an environment that shakes its contents like a snow globe. Originally, planes used an electronic pump system that moved waste along with a deodorizing liquid called Anotec. That method worked, but carrying the Anotec was undesirable for the same reasons as storing water: It raised fuel costs and added weight to the aircraft that could have been allocated for passengers. (Not surprisingly, airlines prefer to transport paying customers over blobs of poop.)

Beginning in the 1980s, planes used a pneumatic vacuum to suck liquids and solids down and away from the fixture. Once you hit the flush button, a valve at the bottom of the toilet opens, allowing the vacuum to siphon the contents out. (A nonstick coating similar to Teflon reduces the odds of any residue.) It travels to a storage tank near the back of the plane at high speeds, ready for ground crews to drain it once the airplane lands. The tank is then flushed out using a disinfectant.

If you’re also curious about timing your bathroom visit to avoid people waiting in line while you void, flight attendants say the best time to go is right after the captain turns off the seat belt sign and before drink service begins.

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Why is Friday the 13th Considered Unlucky?
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Today, people around the globe will feel uneasy about getting out of bed, leaving their homes, or going about their normal daily routines, all because of a superstition. These unfortunate folks suffer from paraskavedekatriaphobia, a common neurosis familiar to us all: the fear of Friday the 13th. But just where did this superstitious association come from, and how did it catch on?

The truth is that no one is absolutely sure where the idea that Friday the 13th is unlucky originated. Donald Dossey, the founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina, suspects the fear can be traced back to a Norse myth about 12 gods who had a dinner at Valhalla—the fabled hall where legendary Norse heroes feasted for eternity after they died—that was interrupted by a 13th guest, the evil and mischievous god Loki.

According to legend, Loki tricked Höðr (the blind god of winter and son of Odin, the supreme god in Norse mythology) into shooting his brother Baldr (the benevolent god of summer who was also a son of Odin) with a magical spear tipped with mistletoe—the only substance that could defeat him. Thus the number 13 was branded as unlucky because of the ominous period of mourning following the loss of such powerful gods by this unwanted 13th guest.

For whatever reason, among many cultures, the number 12 emerged throughout history as a "complete" number: There are 12 months in a year, 12 signs of the zodiac, 12 Gods of Olympus, 12 sons of Odin, 12 labors of Hercules, 12 Jyotirlingas or Hindu shrines where Shiva is worshipped, 12 successors of Muhammad in Shia Islam, and 12 tribes of Israel. In Christianity, Jesus was betrayed by one of his 12 Apostles—Judas—who was the 13th guest to arrive for the Last Supper. Surpassing the number 12 ostensibly unbalances the ideal nature of things; because it is seen as irregular and disrespectful of a sense of perfection, the number 13 bears the stigma of misfortune and bad luck we know today.

WHY FRIDAY?

Friday joins in the mix mostly because all of the early accounts of Jesus’s crucifixion agree that it took place on Friday—the standard day for crucifixions in Rome. As Chaucer noted in The Canterbury Tales, "And on a Friday fell all this mischance." Yet perpetuating Friday as an unlucky day in America came from the late 19th-century American tradition of holding all executions on Fridays; Friday the 13th became the unluckiest of days simply because it combined two distinct superstitions into one. According to the Oxford University Press Dictionary of Superstitions, the first reference to Friday the 13th itself wasn’t until 1913. (So despite actually occurring on Friday, October 13, 1307, the popular notion that the Friday the 13th stigma comes from the date on which the famed order of the Knights Templar were wiped out by King Philip of France is just a coincidence.)

The repercussions of these phobias reverberated through American culture, particularly in the 20th century. Most skyscrapers and hotels lack a 13th floor, which specifically comes from the tendency in the early 1900s for buildings in New York City to omit the unlucky number (though the Empire State Building has a 13th floor). Some street addresses also skip from 12 to 14, while airports may skip the 13th gate. Allegedly, the popular Friday the 13th films were so-named just to cash in on this menacing date recognition, not because the filmmakers actually believed the date to be unlucky.

So, is Friday the 13th actually unlucky? Despite centuries of superstitious behavior, it largely seems like psychological mumbo jumbo. One 1993 study seemed to reveal that, statistically speaking, Friday the 13th is unlucky, but the study's authors told LiveScience that though the data was accurate, "the paper was just a bit of fun and not to be taken seriously." Other studies have shown no correlation between things like increased accidents or injuries and Friday the 13th.

And Friday the 13th isn't a big deal in other cultures, which have their own unlucky days: Greeks and Spanish-speaking countries consider Tuesday the 13th to be the unluckiest day, while Italians steer clear of Friday the 17th. So today, try to rest a little easy—Friday the 13th may not be so unlucky after all.

Additional Source: 13: The Story of the World’s Most Popular Superstition.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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