A British security consultant has invented a device that emits a high-frequency sound that can’t be heard by people over age 25. Since young people can hear higher frequencies than older people, this device, called "the Mosquito", emits a high-pitched pulsing noise that’s unbearable only to younger people.
This principle behind it has also been used to create mosquito ringtones that only teenagers can hear.
In these bad economic times, this device could be a boon to employers looking to ensure that the young employees don’t spend more time than necessary at canteens, vending machines or other non-productive locations.