Coming soon to a cell phone near you ... a ringtone that only kids can hear. Compound Security in England marketed a product called mosquito as a deterrent to unruly or unwanted teenagers. The 17 kilohertz sounds is painfully detected by kids, but not by adults who naturally lose the ability to hear such high frequencies.

Now the situation has been flipped. Instead of being used by adults against kids, kids use the sound against grups (er, grownups if you missed Star Trek). It's been reinvented as a ringtone that only kids, and not parents, can hear. Perfect for school use where phones have been banned. Except for the occasional adult who can still detect it.