Light paintings are a fun technique, dating back to the 19th century and used by luminaries such as Pablo Picasso, Mark Rothko and Andy Warhol. By waving a light source, like a flashlight, in front of a camera set for a long exposure, you can seemingly create static paintings of light in mid air.
Your iPhone is both a camera and a flashlight, but you usually need two of them to make a light painting. Now a new web app called Air Pencil is set to change that, allowing you to create incredible three-dimensional light paintings, no external camera required.