The art of choosing a good watermelon at the market has sparked a heated discussion in China's social media.

Farmers choose watermelons for their customers by tapping the produce at a wholesale market in Zouping county, Shandong province.[Dong Naide/For Chian Daily]

The discussion - with thousands of people contributing on Sina Weibo - began when a Chinese social media user posted a picture of a sign at a supermarket asking customers not to knock on the watermelons.

The photo, which went viral, was originally posted by a user nicknamed "Isolated Guardian".

The sign, which was written in Italian, was seen sticking out of a cart of watermelons in what seems to be an Italian market. The photo was widely shared after Chinese media outlets reported that the message was aimed at Chinese customers.

Chengdu Commercial Paper, for example, posted the following blog seeking Chinese commentary:

"An Italian market set up a sign for Chinese customers: 'Dear customers, please do not tap the watermelons again. They really will not respond!!!' If you were there, what do you want to say to the Italian supermarket?"

The post was shared 7,700 times in the first day. Many netizens found the picture amusing and shared photos of themselves listening for a response from watermelons.

"Knocking before eating is the basic respect we show to watermelons, like saying to them: 'Are you ripe? Are you ready to be eaten?" joked one netizen.

"We have been communicating with watermelons for thousands of years. We can hear their life story with a simple knock," another remarked.

While some netizens seemed to have fun participating in the melon musings, others of more serious mind wanted to know the truth behind the news reports, asking, for example: "Excuse me, which word means Chinese?"

A few netizens expressed anger at the media for spreading rumors.

One responded to the post: "If this sign is aimed at Chinese customers, it should have been written in Chinese. Shame on you."

"Isolated Guardian" removed the post later, claiming the photo of the Italian market was an old one and was not taken by himself.

A quick search online reveals that Chinese shoppers are not alone in tapping on watermelons and listening for a hollow sound. A lot of videos and blogs with advice on how to choose a ripe watermelon can be found on Google.

Melon thumping even appears in an episode of Russian cartoon Nu, pogodi, the BBC reported.

The pinnacle of melon-picking advice may be a smartphone app developed by a group of Chinese students to help a customer identify a good one. The app, called Ting Xi Gua, or "Listen to the Watermelon", picks up the sound when you tap on a melon, analyzes it and decides whether the watermelon is ripe enough.