There is a curious sound with incredible power to be heard only by children and adolescents. It is the hum or buzz mosquito teen. Adults can not hear. And if you do not believe it tested. Make it sound where people are of different ages and check who listens and who does not. Ringing
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Amazing right? People who hear can not believe that others can not hear. And those who do not hear us hard to accept that there is a sound that others can hear and we do not. Do it in groups. I guarantee you surprised faces (starting with oneself) and a fun experience.
The explanation for this is called presbycusis, which is the loss of human ear's ability to attract certain high frequencies (18-20 kHz) after a certain age.
This sound was invented for use in some stores in England to chase the kids, without disturbing the adults, and thus they were hovering around the facilities without buying anything. But after young people used it as a sound in his cell, and thus they can realize they received a message without the teacher listens, and what was originally invented to be used against you, it became something in their favor. So is technology. Some invent, others take advantage.
And you. Do you still listen? ("Old me?, Old hills)