Image copyright Hit-Point

Over the past few weeks, gamers in China have become obsessed with a free-roaming Japanese frog.

Travel Frog has topped the charts in the free game category in Apple's App Store in China for more than a fortnight.

The game, developed by Japanese company Hit-Point and originally called Tabikaeru, is only in Japanese, but is easy to play despite the language barrier.

It lets players own a cute little green frog which lives in a hut, where it eats, writes, reads and sharpens his pencil - sometimes it dozes over books.

Image copyright Hit-Point Image caption The frog sometimes dozes over books

Out in its garden, player can collect clover, the primary currency in the game. You collect 20 clovers once every three hours simply by swiping across the garden, or you buy them with real money if you are too impatient to wait for them to grow.

But the most curious feature of the app is that beyond that, players have very little control of the frog. It will frequently leave its home and travel around Japan on a whim.

Players never know when the frog sets off, when it will get back nor what it will bring on its return. Sometimes the frog goes home within a few hours; sometimes it could be gone for as long as four days.

It might send postcards, clovers, souvenirs - or might do absolutely nothing for its owner.

There is no way for the owner to control or interact with the frog.

Image copyright Hit-Point Image caption Players never know when the frog sets off, gets back or what it brings back upon its return.

The only thing that players can do is to prepare food, tools and amulets for the wandering frog.

Taste of parenthood

"I like the game because the frog does whatever it likes and I don't have to spend a lot of energy on it," 27-year-old Shen told the BBC.

Xian told the BBC she became a frog keeper a week ago, when she saw her friends share photos on WeChat, the Chinese equivalent of Facebook.

"I check my frog almost every 10 minutes at work, because my job is boring. I marvel at the photos that it sends to me from its adventures," said the 25-year-old.

Image copyright Hit-Point Image caption The frog travels and sends photos to the owner

"My mother longs for my return home when I am away, but she wants me to go out when I am home. That's my exact feeling towards my frog," Xian said.

"But I feel desperate when I keep receiving photos of itself: it is so antisocial and doesn't make friends!

"Today, it posed together with a rat, I almost cried with joy, it finally has friends!" Xian said.

Image copyright Hit-Point Image caption The frog could be gone for as long as four days

According to the latest data obtained by the BBC, via US-based App Annie, Travel Frog has been downloaded more than 3.9 million times in Apple's App Store in China since its launch in December.

Chinese players have spent more than $2m (£1.4m) on in-app purchases for their frog babies.

In Japan, however, it's been downloaded a mere 400,000 times on the App Store and Google Play combined, with users spending only $100,000.

So what's the appeal?

"It really suits the post-90s generation, because we are overwhelmed with work," said Shen.

When he saw the game going viral on Weibo, the Chinese equivalent of Twitter, he downloaded it instantly and now checks his phone frequently for frog updates.

Image copyright Hit-Point Image caption It is very rare for owners to receive photos of the frog with a bunch of friends

"Every time I open it, I am full of expectations. I want to know whether my frog is travelling, and what photos it has sent to me. I feel it is my son."

When the frog is travelling, Shen finds another way to kill time.

He has created a WeChat group called "Post-90s Empty Nester Huddle Together for Warmth". Young people share feelings of parenthood in the group, to while away their frogless hours.

Image copyright Reuters Image caption Former Chinese President Jiang Zemin is seen yawning during the opening session of the 19th National Congress of the CPC

As with so much in China, a political angle to the game has evolved too.

Some players have related the love of the frog to "toad worship", the surprising fandom around Jiang Zemin.

The former president Jiang led the Communist Party from 1989 to 2002, and has unexpectedly become a popular figure among young Chinese in recent years, even among those who were not born when he was in power.

His appearance has earned him the nickname "toad".

"I name my frog 'the elder'", Lin Xi, a student from China's South West University posted on Weibo, using another common online term for Jiang.

Image copyright Hit-Point Image caption Many players have received photos of the frog camping with a butterfly

But while the Communist Party may frown on toad worship, they seem so far to be OK with the frog fans.

The People's Daily, the Communist Party's mouthpiece has used it as a chance to encourage "core values" by urging young people to visit their parents more.

"The travelling frog is like everyone away from home," it posted on Weibo.

"What's the feeling of waiting for your kid? Please remember to visit your parents, all wandering frogs."