If you hold a sheet close and blow on it, you'll see the famous face of Marilyn Monroe. Blow on another sheet and a word will appear.

A new iridescent plastic that reveals hidden images with a breath is described in a recent paper published in Advanced Materials. Researchers at the University of Michigan hope to use this technology for anti-counterfeiting purposes, replacing the ubiquitous hologram stickers used on things like luxury handbags and passports with a humidity-activated logo (or celebrity).

Like peacock feathers and butterfly wings, the sheets are iridescent because they are covered with tiny regular structures that diffract light. Their surface is studded with a grid of columns, called nanopillars, each 100 times thinner than a human hair. Previous generations of nanopillars were extremely fragile, breaking when handled or rubbed. By using a blend of polyurethane and epoxy instead of a brittle material like silicon, the researchers were able to make the sheets flexible and durable enough to survive a trip to market.

When a peacock gets wet, it loses its shine. Water droplets scatter incoming light, destroying the intricate interference responsible for its shimmering colors. The researchers accidentally rediscovered this phenomenon when one of them breathed on an iridescent sheet and it became more transparent. To take advantage of this, they used a customized inkjet printer to deposit a thin water-repellant coating in the shape of an image—Marilyn's face, above. When you breathe on the sheet, water condenses on the sheet and makes it transparent—everywhere but on the outline of her face.

"What you see in these images is just the beginning," says study author Nicholas Kotov. By adding layers of nanoparticles with interesting optical properties, Kotov hopes to produce sheets that look distinctive and are hard to replicate, at least without state-of-the-art equipment. The difficulty of making these sheets gives them an advantage over current anti-counterfeiting measures—at least for now. But today's state of the art is in tomorrow's desktop fab lab, and the cat-and-mouse game with counterfeiters is sure to continue.