Counterfeiters and money minters constantly try to outsmart each other. But money could become much harder to forge - thanks to butterfly wings. Butterflies that flit through tropical forests often have brightly colored wings that iridescent in the sun. But it's not pigments that create those eye-catching shades. It's microscopic structures on the insect's wings that reflect the light. Researchers at the University of Cambridge studied an Indonesian butterfly known as the peacock or swallowtail. Scales on the wings are made up of tiny structures that researchers say resemble the inside of an egg carton, with alternating layers of cuticle and air. The light bounces off the structures so that the scales appear to us as a shimmering green. But using optical equipment that can polarize light, those scales appear bright blue. Researchers used nanofabrication techniques to create scales structurally identical to the butterflies, and the structures presented the same gorgeous color. The research appeared in the journal Nature Nanotechnology. Using the "now it's green, now it's blue" technique that the butterflies have evolved, the scientists say we might be able to design money or credit cards that are much harder to copy. And perhaps add colorful flair in the process.