Blind boxes－small, sealed packages containing one of 12 collectible toys in a series－are becoming big business in China, with young adults jumping onto the novelty bandwagon.

During the first hour of the annual Singles' Day online shopping promotion on Nov 11, sales by the Pop Mart flagship shop on Alibaba's Tmall e-commerce site exceeded those for the whole day last year.

A leading collectible toy company in China, Pop Mart sold out its entire stock of 55,000 Labubu dolls, a mini blind box figure, on the platform in just nine seconds, Tmall said in a report.

Pop Mart sold about 4 million Molly dolls, another collectible blind box figure, last year and expects to sell twice as many this year, Wang Ning, who founded the company in 2010, said at the China Central Academy of Fine Arts in April.

Blind boxes have, in recent years, gone from being favorites of children and students to a big "hole" that young adults keep jumping into.

Collectors buy the sealed boxes from stores, vending machines or online vendors for an average of 59 yuan ($8.41). The attraction of the blind box is that the buyer never knows which specific figure they have bought until they crack open the packaging.

"The most charming thing about blind boxes is the gap between what you expect and what you get," said Wen Xi, a former blind box aficionado who works in internet education in Beijing. "We describe it as a hole because new products emerge every year and you can never fill in the hole."

Eleven of the figures in each series are depicted on the boxes, but only an outline of the 12th, "hidden version" is shown. The probability of getting it is less than 1 percent.

"The blind box economy takes advantage of the gambler psychology－vying for a more valuable 'hidden version' by spending less than 100 yuan," said Liu Chunsheng, an associate professor at Central University of Finance and Economics in Beijing.

"Young people are prone to pursue the excitement brought by uncertainty and higher risks, which leads to impulse buying."

Wang Xinge, a 26-year-old college teacher in Beijing, said her fascination with blind boxes took a while to develop.

"I sniffed dismissively at the blind box when I first heard about it from my elder sister, thinking how childish it was," she said. "But my sister liked them, so I thought of buying one as a gift for her when we shopped in Sanlitun in 2015.

"We wanted a cute fawn but missed out, so we tried several times to get the right one. That's when I understood my sister's fascination and jumped into the 'hole'.

"The excitement of getting the exact figure you want is similar to winning a lottery. I could also pay more to buy it online, but that's less thrilling."

While some collectors aim to collect every doll in a series, Wang said she only sought out those that appealed to her, "but sometimes you have to buy seven or eight dolls before getting the one".

Wen, 32, said, "The blind box is both affordable and unaffordable; a single one is cheap, but it encourages impulse buying."

Pop Mart sells its own designs and fashionable imports online and in 120 shops and 600 vending machines in 52 cities in the Chinese mainland.

The origin of blind boxes can be traced back to lucky bags sold in Japan from the late 1860s. In the 1970s, figures from Japanese cartoons began to be sold in vending machines in that country.

In China, children collected cards featuring ancient heroes that were hidden in packs of instant noodles in the 1990s, with blind boxes taking off in 2016, when Pop Mart began using them to sell Molly dolls.

Wang has bought more than 100 blind boxes since she began her collection in 2015. She keeps her 70 favorites in a closet and has sold or exchanged duplicates on online flea market platform Xianyu.

In a report released in June, Xianyu said 300,000 blind box collectors made deals on its platform last year, up 320 percent from 2017. The most popular box is the "hidden version" of Pop Mart's Satyr Rory Sweet Christmas series, which is listed at 2,350 yuan, nearly 40 times its original price in vending machines.

Wen has jumped out of the "hole", having sold most of his collection on Xianyu in 2017 and 2018. He earned 45,000 yuan from dolls he bought for 20,000 yuan.

Anyone can be a blind box expert if they put in the effort and money, he said, but it costs too much time and energy, adding that he once waited for hours beside a vending machine for the chance to get the "hidden version" of a Sonny Angel doll.

"The time and energy you exert doesn't get paid back, that's the main reason why I quit," Wen explained.

Members of Generation Z, those born after 1995, account for most of the 200,000 consumers who spent more than 20,000 yuan last year collecting blind boxes, Tmall said in a report released in August.