Go to this site and listen.
Apparently some UK shopkeepers, trying to control the teen traffic and gathering in front of their shops, setup a speaker that emits a high-pitch tone that only people under the age of 20 can hear.
According to NPR, the teens have struck back and have begun to use the tone for text message alerts without adults knowing what is going on. So wonderfully subversive.
Hat tip to my nephew Greg for alerting me to something I cannot hear (which gives him and his friends so much pleasure).