Some time ago, I came across this mp3 called "mosquito tone." It is a recorded tone at a frequency just high enough to be out of reach for most ears past the age of about 18 years old. When I first read the explanation, I didn't believe it. However, after testing it out on my three daughters, I'm a believer. As if that's not funny enough, the story of how it is being used is pretty funny too.
The story goes that the sound was designed to be used by merchants as a way to keep teenagers from loitering around their stores. They should've known that the tech generation would find a use more to their own advantage. Young people have commandeered the tone for use on their phones and wireless devices. Using the Mosquito tone, kids can receive calls and text messages in class without the teacher's old ears ever getting the signal. Clever! Devious, but clever! You can read more details about how it works here.
Call the younger people in your house into the room and test it out for yourself. It's like taking the hearing test. They'll love that they can hear it and you can't. Enjoy!