A new craze is sweeping through the ranks of Shenzhen's teenagers. Whether it is in school, at the shopping mall or the KTV club, there's only one way to prove you are a real 'Shenzhener' - by speaking Cantonese.

In the past couple of years, there has been growing concern that regional dialects are being lost to the relentless tide of Putonghua. But in Shenzhen, many immigrants are swimming against the current.