A LED throwie is a small LED attached to a coin battery and magnet (usually with conductive epoxy or electrical tape), used for the purpose of creating non-destructive graffiti and light displays. They were devised by the Graffiti Research Lab as a new kind of graffiti art to be used on ferromagnetic surfaces. Part of GRL’s original campaign involved distributing throwies to a group of people, who were then encouraged to throw them onto a metal sculpture, called Alamo, located at Astor Place on the isle of Manhattan in New York City.

- Wikipedia

Watch the LED Throwies in action and also the video on how to construct one … or a bunch of them !