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Why Are Manhole Covers Round?

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If you’re in the habit of looking down at your feet as you walk, you may have noticed that most manholes, and their covers, are round. Or perhaps you’ve been in a job interview where your prospective employer asked this question (which apparently happens enough that it was included in the third edition of The Complete Idiot’s Guide to Job Interviews). He wants to know about your ability to think on your feet, and isn’t so much interested in the correct answer, but it is a good question. Of all of the potential shapes for manholes, why was a circle chosen?

There are a number of reasons, according to Larry Scheckel in his book Ask A Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works. Manholes are round because “it is the best shape to resist the compression of Earth around it.” Round shapes are easier to manufacture than square or rectangular shapes, and because manhole covers are heavy, being round makes them easier to move from place to place (just roll them!). As an added benefit, workers don’t need to line up the covers with any angles, making round covers easy to slip into place.

But perhaps the biggest reason that manhole covers are round is that round covers can’t fall through a circular opening. “For all manholes, there is a ‘lip’ around the rim of the hole, holding up the cover, which means that the underlying hole is smaller than the cover,” Scheckel writes. “A round manhole cover can’t fall through its circular opening, because no matter how you position it, the cover is wider than the hole. But a square, rectangular, or oval manhole cover could fall in if it was inserted diagonally into the hole,” which would be bad news for unobservant pedestrians and drivers alike.

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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.

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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Big Questions
Why Did We Start Wearing Pants?
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It’s a question that has plagued Donald Duck for decades: Who decided pants were necessary? Did the motivation stem purely from modesty, or was there another reason we started climbing into trousers?

Over at Discover, author Sarah Scoles has offered a plausible explanation by describing a 2014 archaeological find in China’s Tarim Basin. Researchers with the German Archaeological Institute excavated what is believed to be the oldest example of pants ever unearthed, made from wool and dating back 3000 years.

The pants themselves held no clue as to why they were made, but their location did. The research team found them buried at the Yanghai cemetery along with a number of other artifacts, including horse-riding gear that was in the same grave: a wooden bit, a bow, and an axe. The pants-wearer was surely someone tasked with galloping around and slaying animals for food—likely necessitating apparel that would allow him to mount a horse without being encumbered by clothing.

A screen shot of Donald Duck near a door
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That idea eventually bled into Greek and Roman culture, where those on horseback sought out a comfortable and practical way of avoiding chafing. (The grave’s proto-pants also appeared to be an early example of being fashion-conscious. While mostly practical, each leg had cross-stitching that appeared to be purely decorative.)

Whether the Yanghai discovery is considered the earliest example of pants depends on how one defines pants. Ötzi, the European iceman first discovered in 1991, lived roughly 5300 years ago and died wearing goatskin leggings. We know a cartoon duck who has a lot of catching up to do.

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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