Today I learned that if you plug in your earbuds or headphones into the microphone jack, they will start functioning as microphones. If the jack takes stereo input, they will act as stereo microphones (the Line In jack, usually colored blue, works nicely for this). At first I thought there was no way this could be true, but after some testing it turns out that both earbuds and headphones can be coaxed into behaving like (very bad quality) microphones.
It then occurred to me that if I could wear my earbuds inside out, I could make very accurate binaural recordings with them. Binaural recordings are stereo recordings that put a microphone at each ear, such that if you listen to them with headphones, your brain will automatically reconstruct the sound sources as if you were really there (example). I wasn't sure if the sound hints would be lost in noise, but surprisingly I was able to come up with working binaural recordings. Despite being fairly low quality due to the amount of noise that had to be removed, the binaural aspect still works. Headphones will give you substantially higher quality, but in order to accurately record them, you must wear them (or otherwise position them) so that the speakers are facing outward. My headphones don't let me do this, so I was only able to verify that they gave significantly higher quality recordings. Predictably, the higher quality the headphones, the better recording you'll get, although it still won't be anywhere near as good as a real microphone.
Shaking a chain around:
Shaking a bunch of keys around:
You can wear earbuds inside out by looping the cord around the back of your ear, and then anchoring the earbud in the reverse direction. While this configuration would fall out normally, since the earbud is now suspended from the top of your ear, it works quite nicely. While the recordings only work for fairly loud noises, its still a fun way to play around with the aspects of binaural sound without needing to buy two microphones and set things up. Use an audio editor to maximize the recording and remove the (very large) amount of noise to get a working recording. Make sure your sound source is very sharp, so its easy for the earbuds or headphones to pick up. If you think binaural recording is cool, considering getting a set of real microphones for high quality recordings (even a pair of $5 microphones will sound better than using your headphones backwards). Professionals will use a dummy head in an effort to fully capture the HRTFs the brain uses for reconstruction.