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Courtesy of MGA Entertainment

Kids love surprise presents. So it’s fitting that last year's holiday toy craze was an egg that hatched a mysterious creature known as a Hatchimal, and the followup this season is the Hatchimals Surprise, with not one but two creatures inside.

It should also come as no surprise, then, that the 2017 holiday shopping season has an early hit in the form of L.O.L. Surprise!, another “blind pack” toy with a huge online following and (sorry, parents) a tendency to sell out as soon as they hit store shelves.

The big appeal of L.O.L. Surprise is that you don’t know what you’re getting until you unwrap the package. Each is sold as a ball with mysterious items inside, including collectible dolls. Each doll comes wrapped in seven layers of packaging that you peel back bit by bit to reveal her and her collection of tiny accessories—little trinkets like doll clothes, stickers, secret messages, fake tattoos, and charms.

Courtesy of MGA Entertainment

The L.O.L. Surprise comes in round packaging that look like a bath bomb, and it even has a line extension called L.O.L. Surprise Fizz that fizz when you drop them in water to get at the little toys inside. The limited-edition L.O.L. Surprise Big Surprise, which includes 50 surprises in a glittery gold ball, debuted just over a month ago and sold out pretty much immediately.

The L.O.L. “Big Surprise” includes 50 surprises in a glittery gold ball. Those trinkets are packed into a series of smaller balls that have to be popped open, unwrapped and/or dropped in water to reveal the surprises inside. It’s out of stock at Toys “R” Us, Target and Walmart, although you can find third-party resellers hawking them — for well over their $69.99 MSRP — on Amazon. Sellers have been asking $120 and up lately on the site for the "Big Surprise," a $50 markup. There are dozens of listings at eBay too, with asking most prices in the range of $110 to $150.

MGA Entertainment, which makes and markets the L.O.L. Surprise, recently added L.O.L. Surprise Pets to the family. The series includes puppies, kittens, bunnies and “ultra-rare” hamsters.

Courtesy of MGA Entertainment

Experts say the appeal of “blind pack” toys like L.O.L. Surprise is driven by the weirdly mesmerizing phenomenon of watching people open things online. “I think it goes to that whole unboxing trend that you see on YouTube,” says Gerrick Johnson, an equity research analyst at BMO Capital Markets. “It’s all about the collectibility and surprise.”

Some of these videos have been viewed millions of times, like this one:

Consumer psychologist Kit Yarrow agreed. "There truly is something to the unboxing and haul videos,” she told Money. “It’s the new version of having your friends over to see what you bought.”

Juli Lennett, toy industry analyst at The NPD Group, says MGA Entertainment hit on the “secret sauce” with the L.O.L. Surprise dolls: The small size, low price point (individual ones start at around 10 bucks each) and wide variety all make the L.O.L. dolls eminently collectible.

Yarrow says the rabid popularity of these toys is fueled by the social nature of how customers amass them. The toys are collected, opened with great excited, and sometimes traded — and all of that is shared online.

Courtesy of MGA Entertainment

“Besides the obvious vicarious thrill of anticipation and discovery, I think we connected to others and part of something when we watch,” she said, adding that it’s a medium ready-made for ever-shrinking attention spans. “Waiting, unveiling, anticipating and all the excruciating pleasures that accompany those activities are even more exciting.”

That is, if you can find them in the first place.

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