Flash rocks, also known as quartz, are piezoelectric rocks! They are one of the few minerals with properties that will create an electrical charge when subjected to mechanical stress.
Put a rock in each hand and forcefully rub the rocks together. To see the glowing electrical effect fully, you should do this in a darkened room. Please note: they do give off a distinctive odor from the friction. Do not hit the rocks together, just rub!
* Do not hold close to eyes while performing the experiment!