The ringtone teachers couldn't hear

In 2005, Welsh inventor Howard Stapleton created the Mosquito: a device emitting a piercing tone at around 17,000 hertz, designed to disperse groups of teenagers loitering outside shops. That was the whole point — only the young could hear it.

It works because of presbycusis, the natural wear of the ear. The cells that pick up the highest frequencies are the first to die, and they never grow back. Past the age of 25 or so, most people stop hearing those tones entirely — without ever noticing they've lost anything.

Teenagers took no time to flip the weapon around: they turned the tone into a phone ringtone, the famous Teen Buzz, which rang in class while the teachers heard nothing at all. The anti-teen device ended up as the teens' best ally. The invention, incidentally, won an Ig Nobel Peace Prize. Richly deserved.