Especially creepy when sticking two of them on the eyes of a statue.

ATTENTION: For red, yellow and orange use a 120 Ohm resistor. For blue, green or white no resistor is needed.

Wikipedia: An LED throwie is a small LED attached to a coin battery and a rare earth magnet (usually with conductive epoxy or electrical tape), used for the purpose of creating non-destructive graffiti and light displays. Artists use them by throwing individual LEDs onto metallic objects, like public sculpture or road infrastructure. By throwing LEDs onto an object, the object itself acts as a canvas. LED throwies were invented in 2006 by the artists Evan Roth and James Powderly the founder of Graffiti Research Lab at Eyebeam Atelier open lab NYC. After Graffiti Research Lab posted the instructions how to make a throwie on Instructables.