They sing it at day care: Baby shark, doo doo doo doo doo doo.
They sing it in their rear-facing car seats: Mama shark, doo doo doo doo doo doo.
They sing it at bath time and dinnertime and way past bedtime: Daddy shark, doo doo doo doo doo doo.
Parents of children under 4 know exactly what this is about. For everyone else, welcome to the inescapable cultural phenomenon that is "Baby Shark."
The catchy children's song has gone explosively viral, saturating the internet and the neural circuitry of toddlers the world over. It's the latest pop culture trend to captivate a young audience - perhaps the youngest ever - fueled by animated, K-pop style YouTube music videos that have racked up more than 3 billion views.
The tune itself isn't exactly new: It started as a simple nursery rhyme more than 20 years ago, a go-to singalong on school playgrounds and summer campgrounds. In 2015, it was reinvented as a series of whimsical music videos by Korean children's entertainment brand Pinkfong. The most popular version features a lineup of cartoon sea creatures alongside singing and dancing Korean children.
For the song's littlest fans, it is an irresistible, fantastical delight. For children's entertainment marketers, it's one more leap toward the globalization of childhood consumerism - mesmerizing prekindergartners who watch again and again on tablet and phone screens.
For parents, it's a maddeningly infectious earworm that haunts at all hours.
"It was playing over and over again in my mind the other night," says Lauren Astor, the mother of 4-year-old and 2-year-old boys in Los Angeles.
"I had to wake my husband up in the middle of the night because I had been dreaming and singing it in my sleep and couldn't stop," says Emily Hassenstab, an Omaha mother of a 2-year-old son.
Caroline Guthrie has been hearing it constantly since her 3-year-old daughter, Dottie, discovered it last year in the sidebar of recommended videos on YouTube Kids.
"For a while she was obsessed," Guthrie says. "There were all these different versions, and she'd want to watch them one at a time in sequence. It was stuck in our heads all the time. My husband would be at work whistling 'Baby Shark.' It infiltrated us. It's become part of our household language."
The song has been a massive success for Pinkfong and its parent company, SmartStudy, which has created more than 100 videos based on "Baby Shark." The most popular version alone has been viewed more than 1.6 billion times on Pinkfong's YouTube channel, according to Jamie Oh, director of global marketing and partnerships at SmartStudy.
The tune, translated into 11 languages, has climbed to first place in children's music on iTunes, Apple Music, Google Play and Amazon, Oh says. It has also inspired an onslaught of spinoff home videos shared under the hashtag #BabySharkChallenge, where people record themselves performing the song and dance.
Celebrity parents are in on the trend, too. References to "Baby Shark" have popped up on the social-media feeds of Jimmy Fallon, Cardi B, Chance the Rapper, Tyra Banks and Kylie Jenner. Those widely viewed posts on Twitter and Instagram gave yet another boost to the easily shareable song's worldwide popularity.
"A simple song that isn't tied too radically to a given culture can, with the right technology, easily become global," says Gary Cross, a cultural historian at Penn State University who specializes in modern childhood. "Children are not yet fully embedded in a culture, and they're very adaptable to fads."
This was true long before YouTube and iPads placed the latest craze at a toddler's fingertips.
Consider the "Happy Birthday" song, Cross says. It was created by two Kentucky schoolteachers in the late 1800s and transported across the country and around the world through word of mouth, film screens and radio.
"The two countries that have been most able to globalize this phenomenon of children's commercial popular culture are America and Japan - and now apparently Korea, which is sort of an extension of that," Cross says. "From the time of Hello Kitty, and products like Nintendo, which really hit the streets in America in 1985, there (has been) a strong acceptance now of all things Asian."
These fads are successful because rather than focusing on instructive or traditional toys and products, they embrace a more playful, imaginative fantasy world for children, says Cross. But when the target audience is so young, it's helpful for parents to be on board.
"Parents want to share their own childhoods with their children, but they also want to share with their children what's new and novel," he says. "So the most appealing product is often one that combines the two, something that's a throwback, but also innovative."
"Baby Shark" falls in that sweet spot: For some millennial parents, the tune and its accompanying dance moves immediately brought back old memories.
"I remember singing it at basketball camp," Astor says, though the version she knew was more dramatic and featured a panicked surfer.
Joana Munson recalls a grislier iteration, too - where the surfer is gradually devoured - back when she sang the song to prekindergarten summer campers nearly a decade ago. "I thought it was so funny that we were singing this dark song to little kids," she says.
She had forgotten about the song by the time she heard it again a couple months ago, when her 2-year-old son suddenly sang the Pinkfong version to her while he was taking a bath.
"I got a big kick out of it," she says. "I was, like, here's a thing that I remember, and now you like it, and this is so cool."
Beyond the nostalgic appeal, these parents are also pleased to see their toddlers already becoming global citizens.
"One of the things I'm excited for is my kids are going to have diverse cultures influencing their perspectives from a very early age," Astor says.
That might make those middle-of-the-night doo doo doo doo doo doo's easier to bear.
"Is it annoying? Maybe," Munson says. "Is it also one of the least harmful things that we have on our planet right now? Absolutely."
Childhood attachments - even silly ones - are formative, she says, and now her son will have something in common with kids who grew up halfway around the world. "And that makes our world a lot smaller."