The hottest iPhone app in town isn't a game or a fancy new way to play music. It's a program that produces just a single beep.
The reason for its popularity is that the noise is identical to the one made when an Octopus card is tapped on a reader.
The 'dood' sound is generated when the user opens the app and presses the screen or when the phone's front camera lens is covered.
Dubbed the baat dat loong simulator from the card's Cantonese nickname, it has become one of Hong Kong's most downloaded apps since it was introduced last week.
Software company Play More, which developed the app, insists it is not designed to cheat traders.
'It's not a billing system but just a piece of software offering people a good laugh,' it says, adding in a legal disclaimer that it will not bear responsibility for any loss caused.