In 2006, Scott Bell earned a spot in the Guinness Book of World Records for the longest-ever barefoot walk over hot coals—250 feet of glowing hot embers, at 1,200 degrees Fahrenheit. Eight months later, he smashed that record with another fire-walk, this time 326 feet.

Now, he runs an events company in the United Kingdom. guiding other people over hot coals and the occasional bed of broken glass as part of corporate team-building exercises and charity events. But walking just six-and-a-half feet over 2,000 Lego pieces? Bell usually gets someone else on his team to do it.

“Out of the three that I do on a regular basis, it’s before I step on the Lego that I think ‘Oh, this is going to be a bit uncomfortable,’” he says, laughing.

Lego walking is increasingly popular at charity events, Lego-themed events, team-building workshops, on YouTube, and even in cabaret sideshow acts. It is exactly what it sounds like: stepping barefoot on a pile or path of Legos, usually of all different sizes. But unlike fire-walking or even glass-walking, walking over a bunch of Legos actually does hurt. Why? And an even better question—what do we get out of it?

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Lego, the stackable plastic brick that we all know and love, debuted in 1958 and since then, people who are around small children have experienced the stupidly painful shock of stepping on a stray Lego barefoot. By this century, it was enough of a recognized thing that, according to Know Your Meme , the utterly vindictive phrase “I hope you step on a Lego” became popular in chat groups and comics from about 2009.

But the first intentional Lego walks started to pop up on YouTube about four years ago. In June 2014, a Portland, Maine, video store ran a promotion: Brave the 12-foot-long “Lego Firewalk” and get The Lego Movie at half-price. The promotion lasted only an hour and a few dozen people, including kids, did it, but Star Trek ’s George Takei posted a picture of the Firewalk and a link to the store, Bull Moose, on his Facebook page. Within a few days, the picture had earned more than 186,000 likes and was shared more than 76,600 times (four years later, that figure had ballooned to 257,000 likes and 150,000 shares).

The Firewalk went viral and within a few weeks, other stores and events across the country were hosting similar walks. Sir Troy’s Toy Kingdom in North Canton, Ohio, the largest independent toy store in the state and a kind of regional mecca for Lego fans, was one of the earliest and most-wholehearted adopters. Not long after Bull Moose’s event, the store was contacted by a local library that was hosting a screening of The Lego Movie in the park; they wanted to know if Sir Troy’s could help them pull one off for the screening. The store constructed an eight-foot-long, two-foot-wide board piled high with 40 pounds of Legos, mostly bricks and worth around $1,000.

“Now we’ve got this long walk, where are we going to store it? So we decided to put it out in the store,” says Heather Marks, head of marketing for Sir Troy’s. The Firewalk is now nearly as popular as the working Lego train set that also lives in the store, and it’s a fixture at in-store birthday parties. Because it’s somewhat portable, they can bring the walk to events and festivals; at a recent gathering, more than 300 people did the challenge. Marks also said that they installed a massive commercial sink in the back of the store just to be able to wash the thousands of Legos. People drive from hours away just to walk barefoot over sharp, pointy bits of plastic—or rather, to get their kids to walk over them: “The reception was very warm, because this was every parents’ revenge.”

This year has seen a surge in Lego walking, encouraged in large part by some high-profile world records bids. In January, Russell Cassevah, a Chesapeake, Virginia, vlogger and Lego fan who goes by the name Brainy Bricks , walked 120 feet to raise money for a local charity that provides Legos to children in hospitals. Then, just a few months later, in March, hugely popular YouTube bro-tainers Dude Perfect walked just shy of 147 feet to earn a Guinness Book of World Records award. But even before the month was out, the hosts of Lego’s own YouTube show, Rebrickulous , walked an astounding 1,264 feet, 6 inches, on a spiraling path of Lego pieces, blowing away Dude Perfect’s record. What could Brainy Bricks do but walk the path of plastic pain again?

On April 21, Cassevah – sweat beading on his brow, teeth gritted the whole way – walked an unbelievable 2,737 feet on a square circuit of red, four-by-two Lego bricks at Philly Brickfest, in front of a cheering crowd and a Guinness World Records adjudicator. By the end, his feet were violently red, bleeding, and swollen – “On every brick, there were eight chances for me to hit a corner and it felt like it,” he told YouTube channel Beyond Bricks . “My feet are on fire right now,” he said as a medic wrapped them in bandages.

There hasn’t been another attempt since—Cassevah said he wanted to set the bar so high, no one would try again. But the challenge is being offered on a less massive scale at events across the world. Scott Bell, the British events organizer, has been offering Lego walking for nearly three years, but he says that it’s become a lot more popular over the last 12 months, especially with charity fundraisers. “The charities like it because it sounds safer than the fire or glass walks,” he acknowledges. People, he says, instinctively shy away from those because everyone, at some point, has burned or cut themselves and they know it hurts. “That fear aspect is just so ingrained, they’ve learned all their life that this is going to hurt, even though they know logically that it won’t hurt,” he says. “I think the with the Lego thing, because it is a toy at the end of the day, it’s not as scary... because you don’t bleed or blister with Lego… the risk factor doesn’t seem as high.”

And yet, as Bell says, it’s the one thing he does regularly that really smarts. World record holder Cassevah said, in response to a question about how he prepared for his second walk, “It hurt so bad, there’s no practice.” Sideshow cabaret performer Bazoo the Kloun, a man who juggles balls made of barbed wire and lets people staple dollar bills to his chest, said on Instagram that he tells people all the time that walking on Legos actually hurts more than walking on broken glass, but no one believes him.

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So why does Lego walking hurt where fire and glass walking don’t? The science of physics and anatomy offer some clues.

Bell says that for fire-walking, he and his team use hardwood logs, letting them burn down for about 45 minutes to an hour, until they’re just embers. Though the coals will give a temperature reading of between 930 and 1,100 degrees Fahrenheit, that isn’t the level of heat one feels when walking over them, provided one doesn’t stop for a selfie. Hot coals, Bell says, are very slow conductors of heat, and the time the foot is in contact with them is too little to cause damage. This isn’t to say that there is no risk of burning—Bell says that he endured serious blisters when he did his first world record walk, and in 2016 , more than 30 people suffered burns to their feet at an event led by motivational speaker Tony Robbins. Similar incidents, Bells says, can be attributed to improperly prepared coals.

Glass-walking, which looks incredibly painful and is perhaps most similar to Lego walking, can actually be relatively painless. To prepare a glass walk, the pieces are typically broken to fairly small size, then poured onto the flat surface and patted down to ensure a more uniform walking surface. Once the walker steps on to the path, the glass shifts and flattens further, and the walker is distributing his or her weight evenly over many potentially sharp points – the “bed of nails” effect. This means that no one piece has enough pressure applied to break the skin or even set off the many pain receptive nerves in the feet.

Legos are—for now at least—built from ABS plastic, an extremely hard and durable terpolymer plastic. They’re built to survive intense levels of abuse without shattering: A single two-by-two brick can withstand up to 4,240 Newtons , an unbelievable amount of pressure. That’s equivalent to a mass of around 950 pounds, and it would take 375,000 other bricks stacked 2.75 miles high on top to exert the same kind of pressure.

So when stepping on a single Lego brick, with its sharp corners and pointy bits and no give at all , there’s nowhere for the force to go except back into your very sensitive foot. (And humans’ feet are very sensitive: Despite the fact that we’re standing on them all the time, feet are, along with hands, lips, and genitals, among the most sensitive areas on our bodies, instantly reactive to painful stimuli and touch. The bottom of each foot is packed with up to 200,000 individual sensory receptors , constantly sending information back to our brains and allowing us to unconsciously adjust our gaits and steps as needed.)

That’s why it hurts to step on just one Lego. It hurts less to step on many at one time because it’s no longer all that pressure applied to a single point, but rather shared over many points. It still stings because unlike with glass, which helpfully shifts and adjusts under your feet, achieving an even weight distribution over Lego bricks is unlikely—they just don’t flatten.

“Glass will move as you stand on it, whereas Lego, you’ll get one that will sort of stand proud and refuse to go down,” says Bell. This also explains why children seem to be able to better withstand the Lego walk pain, something that parents the world over have already clocked, simply because they weigh less and therefore are applying less pressure, Bell says.

But there’s another question about why Lego walks are becoming popular: Why would anyone want to subject themselves to walking on sharp, pointy bits of plastic? Why would we do something so painful? Well, one answer is that it makes us better people .

Recently, psychologists have started paying more attention to the function of pain in social relationships and specifically, in a pro-social (as opposed to anti-social) society. In 2014, Dr. Brock Bastian, a psychologist at the University of Melbourne in Australia, conducted a series of experiments designed to examine the question of whether painful experiences can promote social bonding. In one, he asked groups of strangers to hold their hands in icy water for as long as possible, hold a series of leg squats, and eat hot chili peppers. He found that groups that shared painful challenges were more cooperative during an economic game, as compared to those who didn’t go through the painful experiences. His lab’s conclusion was that shared painful experiences can solidify social bonds and can also create trust between people who don’t know each other at all—some of the participants even exchanged contact information after the experiment ended.

In some ways, it seems like the more extreme the painful ritual, the more deeply felt the pro-social benefits. A 2013 study observing the Hindu festival of Thaipusam on the island of Mauritius—which involves celebrants piercing their ears, and the skin on their face, chest, and back, with skewers—found that not only did the people participating in the ritual donate more to the temple after their ordeal than people who didn’t, but people who watched also gave more. Researchers concluded, “Overall, extreme rituals appear to amplify pro-social attitudes and behaviors, and direct or empathic experiences of pain may be the link connecting these ordeals to pro-sociality.” In 2017, another field study examining similar religious rituals found that moral behavior increased among those watching painful and extreme rituals, suggesting that the rituals had “moral cleansing effect on the numerous individuals observing the rituals, which may imply that these rituals evolved to advance and maintain moral societies.”

A neurochemical explanation even undergirds the pro-social benefits of shared, ritualistic pain. During these high arousal states, humans are flooded with all kinds of intense neurotransmitters and hormones, including dopamine, oxytocin, vasopressin and serotonin. Some of these are linked to the creation of social trust and even love—oxytocin and vasopressin have both been popularly (and somewhat myopically) labelled “love” or “cuddle” hormones. Oxytocin is associated with inducing feelings of trust in those around you, reducing fear, and increasing empathy, and serotonin is implicated in reducing anxiety. Meanwhile, dopamine, which is linked with the brain’s management of reward and risk, also makes you feel good about the whole thing. All of this means that evolutionarily, shared painful experiences can stimulate bonding and group cohesion, and create meaning for people.

Of course, Lego walking is nowhere near the level of pain or indeed, social meaning, as pushing a metal skewer through your cheek. And yet, the principal of pro-sociality through a painful experience remains – just witness the crowds cheering Brainy Bricks on at Philly Brickfest, or the way corporations are now using Lego walks as part of team building exercises. Maybe what Lego walking offers us is a way to have our painful rituals at not too great a cost and with a lot of Instagrammable moments. Plus, it’s fun.

“It’s not that risky… and everyone enjoys watching people be a bit uncomfortable,” said Bell, explaining that they seem to especially enjoy it knowing that they’re about to do it themselves: “It’s this, ‘We’re all in it together.’”