A photo posted by a Sina Weibo user shows her listening to the sound from a watermelon. [Photo/Sina Weibo]

"Isolated Guardian" removed his post later on June 19, claiming the photo of the Italian market was an old one and 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 pick a ripe watermelon can be found on Google. Such as this piece on problem-solving website wikiHow, which suggests a ripe watermelon should give a "full sound" rather than a "dull and deep one" if one taps it with the knuckle.

It even appears in an episode of a Russian cartoon called "Nu, pogodi", as BBC reported.

There is also a smart phone app developed by a group of Chinese students to help you find a good watermelon. The app named "Ting Xi Gua", or "Listen to the Watermelon", picks up the sound when you tap on a watermelon, analyzes it and decides whether the watermelon is ripe enough.

Smart phone app "Ting Xi Gua" helps you pick a perfectly ripe watermelon.