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Why Do We Have Fingerprints?

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Though fingerprints are handy for identifying perps, biologically, scientists still aren't quite sure what our fingerprints are for. But as they test different hypotheses, they're getting closer to the answer—and learning some pretty cool stuff in the process.

Is it to improve our sense of touch?

In a 2009 study, researchers from the Ecole Normale Superieure in Paris built two biomimetic tactile sensors, which mimic the human ability to touch and perceive texture. One had grooves that mimicked fingerprints; the other was flat like smooth skin. When these faux fingers moved across roughly-textured surfaces, the fingerprinted sensors produced vibrations up to 100 times stronger than the smooth ones. These vibrations, the scientists found, were dominated by a frequency in the optimal range of sensitivity of the Pacinian corpuscles, receptors in our skin that detect pressure changes and vibrations. These researchers think that our fingerprints' job might be to amplify certain tactile information so that it's more easily processed by the nervous system. They also suggest that the swirling patterns of fingerprints ensure that some of the ridges are always brushing sideways across a surface, no matter which way the finger is moving, to better generate vibrations.

Is it to improve our grip?

Humans, apes, monkeys and koalas all have fingerprints. Some New World monkeys even have ridged pads on their tree-gripping tails. Fingerprints’ design, and their presence in all these animals, has led people to think that they’re an adaptation for improved grip while climbing trees and manipulating objects, but there isn’t much experimental evidence for that. Research by biomechanicists at the University of Manchester, who tested the idea in 2009, suggests that a good grip isn’t fingerprints’ forte. Dr. Roland Ennos and his student Peter Warman tested the grip of Warman's fingers at different angles on strips of acrylic glass sheets similar to Plexiglas. While many solid objects obey Amonton's law and friction between them is proportional to the force between them, the friction between finger and glass increased less than Ennos expected when more pressure was applied. The pair inked Warman's fingers to measure the contact area between them and the sheets and found that friction did increase when the contact area increased, but also noted that the grooves between fingerprint ridges reduce the fingers' contact surface with the glass by about one third, compared with smooth skin, and actually reduced friction and ability to grip.

What are some other possibilities?

Ennos and Warman throw out a few other plausible explanations for fingerprints at the end of their paper: that they allow our skin more to more easily comply with and deform to objects we're touching or holding, reducing shear stress and preventing blister formation; that they increase friction on rough surfaces compared with flat skin because the ridges project into the depressions on these surfaces and provide a higher contact area; that they facilitate runoff of water like tire treads. Ennos says his lab is testing all of these hypotheses, but hasn’t published any results yet.

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Big Questions
What Happened to the Physical Copy of the 'I Have a Dream' Speech?
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AFP, Getty Images

On August 28, 1963, Martin Luther King Jr. stood on the steps of the Lincoln Memorial and gave a speech for the ages, delivering the oratorical masterpiece "I Have a Dream" to nearly 250,000 people.

When he was done, King stepped away from the podium, folded his speech, and found himself standing in front of George Raveling, a former Villanova basketball player who, along with his friend Warren Wilson, had been asked to provide extra security around Dr. King while he was speaking. "We were both tall, gangly guys," Raveling told TIME in 2003. "We didn't know what we were doing but we certainly made for a good appearance."

Moved by the speech, Raveling saw the folded papers in King’s hands and asked if he could have them. King gave the young volunteer the speech without hesitation, and that was that.

“At no time do I remember thinking, ‘Wow, we got this historic document,’” Raveling told Sports Illustrated in 2015. Not realizing he was holding what would become an important piece of history in his hands, Raveling went home and stuck the three sheets of paper into a Harry Truman biography for safekeeping. They sat there for nearly two decades while Raveling developed an impressive career coaching NCAA men’s basketball.

In 1984, he had recently taken over as the head coach at the University of Iowa and was chatting with Bob Denney of the Cedar Rapids Gazette when Denney brought up the March on Washington. That's when Raveling dropped the bomb: “You know, I’ve got a copy of that speech," he said, and dug it out of the Truman book. After writing an article about Raveling's connection, the reporter had the speech professionally framed for the coach.

Though he displayed the framed speech in his house for a few years, Raveling began to realize the value of the piece and moved it to a bank vault in Los Angeles. Though he has received offers for King’s speech—one collector wanted to purchase the speech for $3 million in 2014—Raveling has turned them all down. He has been in talks with various museums and universities and hopes to put the speech on display in the future, but for now, he cherishes having it in his possession.

“That to me is something I’ll always be able to look back and say I was there,” Raveling said in the original Cedar Rapids Gazette article. “And not only out there in that arena of people, but to be within touching distance of him. That’s like when you’re 80 or 90 years old you can look back and say ‘I was in touching distance of Abraham Lincoln when he made the Gettysburg Address.’"

“I have no idea why I even asked him for the speech,” Raveling, now CEO of Coaching for Success, has said. “But I’m sure glad that I did.”

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Big Questions
How Are Rooms Cleaned at an Ice Hotel?

Cleaning rooms at Sweden’s famous ICEHOTEL is arguably less involved than your typical hotel. The bed, for example, does not have traditional sheets. Instead, it’s essentially an air mattress topped with reindeer fur, which sits on top of a custom-made wooden palette that has a minimum of 60 centimeters of airspace below. On top of those reindeer hides is a sleeping bag, and inside that sleeping bag is a sleep sack. And while it’s always 20ºF inside the room, once guests wrap themselves up for the night, it can get cozy.

And, if they’re wearing too many layers, it can get quite sweaty, too.

“The sleep sack gets washed every day, I promise you that. I know it for a fact because I love to walk behind the laundry, because it’s so warm back there," James McClean, one of the few Americans—if not the only—who have worked at Sweden's ICEHOTEL, tells Mental Floss. (He worked on the construction and maintenance crew for several years.)

There isn’t much else to clean in most guest rooms. The bathrooms and showers are elsewhere in the hotel, and most guests only spend their sleeping hours in the space. But there is the occasional accident—like other hotels, some bodily fluids end up where they shouldn’t be. People puke or get too lazy to walk to the communal restrooms. Unlike other hotels, these bodily fluids, well, they freeze.

“You can only imagine the types of bodily fluids that get, I guess, excreted … or expelled … or purged onto the walls,” McClean says. “At least once a week there’s a yellow stain or a spilled glass of wine or cranberry juice … and it’s not what you want to see splattered everywhere.” Housekeeping fixes these unsightly splotches with an ice pick and shovel, re-patching it with fresh snow from outside.

Every room has a 4-inch vent drilled into the icy wall, which helps prevent CO2 from escalating to harmful levels. Maintenance checks the holes daily to ensure these vents are not plugged with snow. Their tool of choice for clearing the pathway is, according to McClean, “basically a toilet brush on a stick.”

When maintenance isn’t busy unstuffing snow from that vent hole, they’re busy piping snow through it. Every couple days, the floor of each room receives a new coat of fluffy snow, which is piped through the vent and leveled with a garden rake.

“It’s the equivalent of vacuuming the carpet,” McClean says.

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