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.
Some users see it differently. 'It can help to cheat the bus fare,' one wrote in a review.
Octopus Cards said yesterday it had not received any complaints.
'If customers use Octopus for payment, the shopkeepers or bus drivers can confirm whether the payment is completed not only by the 'dood' sound,' a spokesman said. 'They can also verify via the light signal on the Octopus readers or display at the cashier terminal or the bus driver's control panel.'
Taxi and Public Light Bus Concern Group chairman Lai Ming-hung said drivers had been warned.