Bone conduction speakers give you the ability to hear things that nobody else can by vibrating your own bones. This covert system -- that fits perfectly inside of a pen -- can be activated by simply biting down on it.
Using a standard FM radio receiver and piezoelectric transducer, creator Ben Krasnow has built the perfect clandestine listening device. Just bite down on the tip of the pen and the speaker inside will vibrate your jaw, sending sound waves up to the cochlea in your ear. Nobody else around you will have any idea that you're listening to something.
Krasnow notes that this could be used to cheat on a test, deliver secret information, or -- as a less "evil" use -- can be placed in a CamelBak for hiking music that won't block out or disturb the surroundings. Check out the whole video to see how he does it.