This is pretty cool: CNN.com - Making a video screen out of thin air - Sep. 15, 2003

SAN FRANCISCO, California (Reuters) -- In a museum in Tampere, Finland, Ismo Rakkolainen's fog machine conjures up the Mona Lisa on an invisible sheet of water particles. Thousands of miles away in Hermosa Beach, California, a graduate student passes his hand through an image of a DNA strand produced -- apparently out of thin air -- by a modified video projector.