Earphones and electric guitars I remember being in my college dorm, watching a film with a friend. His computer wasn't loud enough and we couldn't hear properly. So he took his earphones and put them on a electric guitar coil. He then plugged his guitar to his amp and voilà, we could listen through the amp.
Why on earth did this work? 
 A: Very basically, a speaker driver has a magnet and a coil which move relative to each other when electric current is applied. (The speaker cone is attached to the moving parts and this is what moves the air that causes the audible sound).
When placed close to a guitar pickup, the tiny coil movement itself (not the cone/diaphragm movement) and/or magnetic flux generated by the movement acts on the magnetic field of the pickup in a manner similar to the movement of the guitar strings themselves.
It is also possible, depending on the design of the headphones, to use them as microphones. Many years ago, I used a set of can-style headphones plugged into an reel-to-reel tape recorder as a sort of acoustic guitar pickup.
A: To amplify the guitar strings, the pick-ups sense vibrations of air from the strings as they vibrate when plucked or picked. The earphones were producing air vibrations in the same manner and when placed closely to the pickups, they were able to receive the vibrations and amplify the sounds coming out of them!
