Chameleons normally do their best to blend into the background. But when adult males find themselves confronting a rival, they rapidly switch on bolder colors, changing from green to yellow, for example. Scientists had suggested they may do so by moving pigments around in their skin cells. Today, however, researchers report online in Nature Communications that chameleons change colors by rearranging a lattice of nanocrystals in one of the top layers of skin cells. These cells, called iridophores, contain tiny crystals made from guanine, one of the nucleic acid building blocks of DNA. The nanocrystals have a highly ordered arrangement, which normally causes them to strongly reflect one color of light, such as green. But as shown in this video, when another male enters their surroundings, the animals stretch their skin cells, broadening the nanocrystalline lattice, thereby causing it to reflect a longer wavelength of light, such as yellow. In addition, the team found that chameleons contain a second, deeper layer of iridophores that reflect heat-producing infrared light, which presumably helps the animals stay cool. The new insights could help scientists design novel materials that stretch to change colors.