“It’s not a companionship, but more a feeling of connectedness that players get out of it,” says clinical psychologist Jamie Cheng. She says: “You’re fulfilling a need through the frog: having a simple life outside of the office, going wherever you want without restrictions.”

It also shares many of the features which made its predecessor wildly popular in East and West alike, namely its cute, whimsical aesthetic and intuitive gameplay which makes it accessible to players despite only being available in Japanese.

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In the game, players keep a virtual frog which – unusually, for a game – they cannot control or interact with in any meaningful way. Instead, the frog frequently leaves its small hut on trips around Japan, bringing back souvenirs and photos.

Players can collect (or buy, with real-world cash) green clovers, the in-game currency which buys snacks and items to equip their green companion for its travels. At home in the game, their introverted pet spends however long it wants reading, writing or eating food.

From a behavioural psychology perspective, the game design is hardwired to make players addicted due to its varied-ratio and varied-interval schedule of reinforcement – in other words, how the frog leaves and comes back with souvenir “rewards” at random times.

That makes the game’s psychological reward mechanism the same as that used in casino slot machines, according to Cheng. Much like an optimistic slot machine user, a Travel Frog player, not knowing when the next payout will come, ends up repeatedly checking up on the frog’s latest progress.

“We check our phones regularly to see what it has brought us. That’s one of the most addictive reinforcement schedules that would cause someone to keep playing it,” she says.

Medical student Shelly Liang, 23, says she enjoys the game precisely because it requires so little energy or emotional investment, having first read about it in numerous Chinese-language articles weeks ago. 

Does playing it give her a sense of achievement? “Not at all. For me it’s like an exercise in letting go of things; getting used to not being able to control everything,” she says. “The articles kept mentioning how the design reflected the Japanese philosophy of minimalism and also a sense of solitude, so that’s why I quite like it.”

Unlike many of her friends who are also players, Liang does not feel protective of her virtual pet, and never gets anxious when it is away for a long time. “Some people are really obsessed with finding out which foods make it go places,” she says, “but I don’t really care where it travels.”

Because Hong Kong pop culture is often vastly different from that of the mainland, viral trends north of the border can get a bemused reception in the city. “People were initially [mocking] it – that’s how I started! But since it was so popular on the mainland, I thought I’d give it a go,” Liang says, adding that is how most of her Hongkonger friends started playing. 

For Liang, the game expresses the spirit of so-called “Zen youth”, a growing subculture in Japan and mainland China where millennials give up endlessly striving for achievements or status, and embrace a more relaxed lifestyle. “They’re using this game as an outlet to enact that philosophy, which I quite like,” she adds.

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Melissa Lau, 25, also says she finds the game calming and therapeutic, but claims not to be obsessed with it like many of her friends. Lau, who works in investment, started playing it a month ago after all her friends started discussing it.

“I think it’s okay – not spectacular – but I like to play it just to see where my frog travels all over Japan,” she says. “I like the design and Japanese manga-like graphics. Mostly it’s just a way of passing time.”

She has spent “around HK$20” buying green clovers for her virtual pet.

Cheng, the psychologist, who is a mother herself, says she has even found the game an unlikely source of parenting tips. “I think it’s one of the best ways to teach parents that there are very few things they can control, especially when their kids have grown up. So maybe we should let them be!”