Marcella is eighteen and lives in a Texas suburb so quiet that it sometimes seems like a ghost town. She downloaded TikTok last fall, after seeing TikTok videos that had been posted on YouTube and Instagram. They were strange and hilarious and reminded her of Vine, the discontinued platform that teen-agers once used for uploading anarchic six-second videos that played on a loop. She opened TikTok, and it began showing her an endless scroll of videos, most of them fifteen seconds or less. She watched the ones she liked a few times before moving on, and double-tapped her favorites, to “like” them. TikTok was learning what she wanted. It showed her more absurd comic sketches and supercuts of people painting murals, and fewer videos in which girls made fun of other girls for their looks.

When you watch a video on TikTok, you can tap a button on the screen to respond with your own video, scored to the same soundtrack. Another tap calls up a suite of editing tools, including a timer that makes it easy to film yourself. Videos become memes that you can imitate, or riff on, rapidly multiplying much the way the Ice Bucket Challenge proliferated on Facebook five years ago.

Marcella was lying on her bed looking at TikTok on a Thursday evening when she began seeing video after video set to a clip of the song “Pretty Boy Swag,” by Soulja Boy. In each one, a person would look into the camera as if it were a mirror, and then, just as the song’s beat dropped, the camera would cut to a shot of the person’s doppelgänger. It worked like a punch line. A guy with packing tape over his nose became Voldemort. A girl smeared gold paint on her face, put on a yellow hoodie, and turned into an Oscar statue. Marcella propped her phone on her desk and set the TikTok timer. Her video took around twenty minutes to make, and is thirteen seconds long. She enters the frame in a white button-down, her hair dark and wavy. She adjusts her collar, checks her reflection, looks upward, and—the beat drops—she’s Anne Frank.

Marcella’s friends knew about TikTok, but almost none of them were on it. She didn’t think that anyone would see what she’d made. Pretty quickly, though, her video began getting hundreds of likes, thousands, tens of thousands. People started sharing it on Instagram. On YouTube, the Swedish vlogger PewDiePie, who has more than a hundred million subscribers, posted a video mocking the media for suggesting that TikTok had a “Nazi problem”—Vice had found various accounts promoting white-supremacist slogans—then showed Marcella’s video, laughed, and said, “Never mind, actually, this does not help the case I was trying to make.” (PewDiePie has been criticized for employing anti-Semitic imagery in his videos, though his fans insist that his work is satire.) Marcella started to get direct messages on TikTok and Instagram, some of which called her anti-Semitic. One accused her of promoting Nazism. She deleted the video.

In February, a friend texted me a YouTube rip of Marcella’s TikTok. I was alone with my phone at my desk on a week night, and when I watched the video I screamed. It was terrifyingly funny, like a well-timed electric shock. It also made me feel very old. I’d seen other TikToks, mostly on Twitter, and my primary impression was that young people were churning through images and sounds at warp speed, repurposing reality into ironic, bite-size content. Kids were clearly better than adults at whatever it was TikTok was for—“I haven’t seen one piece of content on there made by an adult that’s normal and good,” Jack Wagner, a “popular Instagram memer,” told The Atlantic last fall—though they weren’t the only ones using the platform. Arnold Schwarzenegger was on TikTok, riding a minibike and chasing a miniature pony. Drag queens were on TikTok, opera singers were on TikTok, the Washington Post was on TikTok, dogs I follow on Instagram were on TikTok. Most important, the self-made celebrities of Generation Z were on TikTok, a cohort of people in their teens and early twenties who have spent a decade filming themselves through a front-facing camera and meticulously honing their understanding of what their peers will respond to and what they will ignore.

I sent an e-mail to Marcella. (That’s her middle name.) She’s from a military family, and likes to stay up late listening to music and writing. Marcella is Jewish, and she and her brothers were homeschooled. Not long before she made her video, her family had stopped at a base to renew their military I.D.s. One of her brothers glanced at her new I.D. and joked, accurately, that she looked like Anne Frank.

In correspondence, Marcella was as earnest and thoughtful as her video had seemed flip. She understood that it could seem offensive out of context—a context that was invisible to nearly everyone who saw it—and she was sanguine about the angry messages that she’d received. TikTok, like the rest of the world, was a mixed bag, she thought, with bad ideas, and cruelty, and embarrassment, but also with so much creative potential. Its ironic sensibility was perfectly suited for people her age, and so was its industrial-strength ability to turn non-famous people into famous ones—even if only temporarily, even if only in a minor way. Marcella had accepted her brush with Internet fame as an odd thrill, and not an entirely foreign one: her generation had grown up on YouTube, she noted, watching ordinary kids become millionaires by turning on laptop cameras in their bedrooms and talking about stuff they like. The videos that I’d been seeing, chaotic and sincere and nihilistic and very short, were the natural expressions of kids who’d had smartphones since they were in middle school, or elementary school. TikTok, Marcella explained, was a simple reaction to, and an absurdist escape from, “the mass amounts of media we are exposed to every living day.”

TikTok has been downloaded more than a billion times since its launch, in 2017, and reportedly has more monthly users than Twitter or Snapchat. Like those apps, it’s free, and peppered with advertising. I downloaded TikTok in May, adding its neon-shaded music-note logo to the array of app icons on my phone. TikTok’s parent company, ByteDance, is based in China, which, in recent years, has invested heavily and made major advances in artificial intelligence. After a three-billion-dollar investment from the Japanese conglomerate SoftBank, last fall, ByteDance was valued at more than seventy-five billion dollars, the highest valuation for any startup in the world.

I opened the app, and saw a three-foot-tall woman making her microwave door squeak to the melody of “Yeah!,” by Usher, and then a dental hygienist and her patient dancing to “Baby Shark.” A teen-age girl blew up a bunch of balloons that spelled “PUSSY” to the tune of a jazz song from the beloved soundtrack of the anime series “Cowboy Bebop.” Young white people lip-synched to audio of nonwhite people in ways that ranged from innocently racist to overtly racist. A kid sprayed shaving cream into a Croc and stepped into it so that shaving cream squirted out of the holes in the Croc. In five minutes, the app had sandblasted my cognitive matter with twenty TikToks that had the legibility and logic of a narcoleptic dream.

TikTok is available in a hundred and fifty markets. Its videos are typically built around music, so language tends not to pose a significant barrier, and few of the videos have anything to do with the news, so they don’t easily become dated. The company is reportedly focussing its growth efforts on the U.S., Japan, and India, which is its biggest market—smartphone use in the country has swelled, and TikTok now has two hundred million users there. ByteDance often hacks its way into a market, aggressively courting influencers on other social-media networks and spending huge amounts on advertising, much of which runs on competing platforms. Connie Chan, a general partner at Andreessen Horowitz, told me that investors normally look for “organic growth” in social apps; ByteDance has been innovative, she said, in its ability and willingness to spend its way to big numbers. One former TikTok employee I spoke to was troubled by the company’s methods: “On Instagram, they’d run ads with clickbaity images—an open, gashed wound, or an overtly sexy image of a young teen girl—and it wouldn’t matter if Instagram users flagged the images as long as the ad got a lot of engagement first.”