Scientists at the University of Southampton in the United Kingdom and the Deutsches Kunststoff-Institut (DKI) in Darmstadt, Germany have developed a new type of self-assembling polymer opal film, which can be used to make objects that change color when stretched. These polymer films are photonic crystals, made up of trillions of little spheres with holes between them similar to certain wavelengths of light. These holes preferentially reflect certain frequencies based on their precise size. Applications for the technology are still being thought up, but counterfeit prevention, smart materials, and food packaging have been proposed.