Screengrab: YouTube/Mackenzie Ziegler

You’ve seen Fingerlings. They’re the colorful monkey, sloth, or unicorn clinging to all those prepubescent fingers. Their sensors and AI allow them to react to gestures and sounds, so they can chat, kiss, snore, and even fart. They’re a hell of a lot cooler than a fidget spinner and at $15 they’re about the same price. But if you want to buy one now you’re probably shit-out-of-luck. Fingerlings are out of stock at almost all big-box stores, and they’re selling on eBay with prices ranging from double the original to as much as $5,000.

That’s all because Fingerlings are the latest volume in the canon of must-have children’s holiday gifts. They’ve joined the historic ranks with Tamagotchi, Beanie Babies, Furby, Tickle Me Elmo, and Hatchimals. But while the gift sensations of your childhood were often flashes of good luck or expensive marketing campaigns, this year’s sold-out toy was socially engineered—from concept to launch—to be a viral phenomenon.

It all started with viral videos of tiny adorable monkeys. “There were videos and photos of pygmy marmosets we kept shooting around internally,” Davin Sufer told Gizmodo. Sufer is the chief technology officer of the Montreal-based WowWee, maker of Fingerlings. “These tiny little monkeys that fit in your hand and they’re super, super cute.”

The pygmy marmoset is the smallest monkey in the world—weighing about 100 grams and teeny enough to fit entirely into a shirt pocket without even a tuft of hair poking out. Sufer showed Gizmodo the photo that first sparked the WowWee team’s interest, a smokey glassy-eyed critter clinging to an index finger, with a look that could be bemusement or fear.

WowWee brand manager Sydney Wiseman was especially fascinated with the palm-sized primate, and has been since she was a kid. Wiseman, niece of WowWee owners Peter and Richard Yanofsky, kept emailing her colleagues photos of the wee creature, insisting they make a robot version. Around May 2016, designers and engineers started designing the toy, trying to figure out if it should be made of cloth, plush, or plastic. By October they had prototypes with their AI and sensors inside.

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WowWee has been in the toy robotics game for a while. One of their most successful previous bots was the MiP, a wheeled freestanding robot that also came in a dinosaur and Minion version. Before that, their major success was Robosapiens, launched in 2004, which walked on two legs and cost about $100. But with Fingerlings, WowWee simplified the technology that it had been putting into larger expensive toys (approximately $40 to $120) and sold it for about $15.

Dufner sounds like Dr. Frankenstein when he describes bringing the Fingerling to life. “The first time we saw it all together in the prototype—all that design and engineering—the blinking eye, the head motion. We started animating it, we hooked it up to a computer and started loading up sound. That moment the whole company had this awareness that this is really big.”

Then the company reached out to young social media influencers, offering to send them a piñata banana filled with Fingerlings (and, in some cases, pay them) if they posted about it on social media using the hashtag #FingerlingsFriday on the day of the US launch, August 11.

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On that day, young YouTubers like Annie and Hope, Miss Jayden B, ForEvaAndForAva, and Jacy and Kacy filmed themselves bashing in a cardboard fruit vessel and playing with gassy robot monkeys, then posted on YouTube and Instagram. Millions of young subscribers and followers found out about the new toy by watching their favorite cool girls on their computers and phones.

But Fingerlings most important coup was Mackenzie Ziegler, a singer and dancer who built her following on the reality show Dance Moms, and now has 8.9 million Instagram followers and 1.8 million YouTube subscribers.

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After the monkeys tumbled out of the battered banana, Ziegler and her friend, Lauren Orlando, giddily played with the Fingerlings, exclaiming over the little adorable robots.

“It really is social media that’s driving this because it used to take weeks, months for things to catch on. Now you know millions of people could see them in a 24-hour period,” Chris Byrne, toy analyst and content director of toy trade publication TTPM, told Gizmodo. “It’s the same as you or I might have taken a toy to the playground and showed it to a friend who went home and said, ‘Oh I want that.’ But now [there are] hundreds of thousands of friends.”

Byrne, who is author of Toy Time!: From Hula Hoops to He-Man to Hungry Hungry Hippos, said it’s becoming more difficult for toy companies to engage young consumers, “You don’t have kids plopped in front of the television at a certain time watching Cartoon Network or Nickelodeon,” he said. “Now the media is so fragmented. Kids are consistently their own programmers. They choose what to watch, and they are likely to be watching things on a mobile devices. They become really fast consumers and sharers of media.”

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According to Byrne, that environment is fertile for viral small toys like fidget spinners, Fingerlings, or last year’s big toy Hatchimals, furry critters that hatch out of eggs. But products that aren’t as Snapchat friendly, like big robots and construction sets, are less likely to take off.

So if you can’t find a precious, squawking monkey to gift a child this holiday season, know that the Fingerling was meticulously engineered—from meme to YouTube sensation—precisely to sell out.