In Chinese kung-fu legends, there is a kind of kung fu where you can make a special sound that only one person can hear. Before this kind of things only happened in stories, but now it is possible because of technology.
A new technology developed at National Taiwan University can allow cell phones, laptops, and PDAs to read special messages hidden in sounds. The sounds are like normal music and cannot be understood by human beings. But the computer can use microphones to pick up the sound messages.
The advantage of using digital sound is that you don’t need the internet to get messages. Any place that can play sounds or music can also allow you receive messages.