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'.