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How to Walk Across Hot Coals

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ThinkStock

People have been scampering across hot coals for thousands of years. A bed of embers can exceed 1000°F, and the world’s hottest firewalk in 1997 actually topped 1750°F—the same temperature used for cremations. But with the right preparation, experts prance across them with barely a blister. Here’s how they do it.

1) Get Wood

A safe walk requires the right coals, usually cherry or maple wood. Hardwoods are excellent insulators, and they’ll protect feet from some of the heat—even when they’re aflame. (That’s why the wooden handle on a saucepan stays cool when you’re cooking.) Cherry or maple embers also glow a daunting red-orange, but they actually don’t burn as hot as other charcoals, like olive or locust woods.

2) Build a Runway

Once the fire has burnt down, rake the coals. This step makes the red-hot landing strip of doom look even more terrifying, but it will actually spread cold charcoal to the surface, adding insulation. Firewalkers also flatten the coals. Patting down a path keeps them from sinking into the sizzling embers, protecting the sensitive tops of your feet.

3) Break Out a Good Book

After making the fire, firewalkers need to kill time for about 20 minutes. Embers that still hold water can transfer heat to feet faster. Letting the coals dry means they won’t sear any soles. Then they sprinkle a thin layer of ash on top. Ash is a terrible heat conductor, and it can block some warmth radiating from the coals.

4) Just Add Water

After waiting for the bed to cool to a balmy 1000°F walkers dip their feet in some water. When liquid meets intense heat, it can form an insulating layer of steam. It’s called the leidenfrost effect, and it’s why you can snuff out a candle’s flame with two wet fingers. The moisture may act as a protective glove for feet.

5) Walk, Don’t Run

Once experts step onto the coals, they walk briskly and don’t stop. Their feet would sink into the ashpit if they run or hard-step. The lighter the stride, the less chance scorching cinders will wedge between their toes. Each step should last half a second or less.

6) Believe in Physics.

Coals may be hot, but they’re terrible at transferring heat. They have a “low thermal capacity.” That is, it takes them relatively long time to bake a walker. (It’s like sticking your hand in an oven set to 400°F. The air feels hot, but it won’t burn you instantly.) As long as they keep moving, each step will absorb very little heat from the embers.

Okay, we’ll admit that even after knowing how firewalking works, it still seems like a terrible thing to put your feet through. Don’t try it. Instead, why not kick up your heels and relax with a cold Dos Equis?

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iStock // Ekaterina Minaeva
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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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May 23, 2017
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