It is a photograph of ice crystals growing on a sheet of very cold glass. There is a polarized filter below the glass, and another that you hold and look through. As you turn the filter, you see colors in the ice crystals. Why? Normally, when you have two polarizing filters and cross them, they block most of the light. Certain materials, such as quartz, calcite, and ice have a property known as birefringence or double refraction. This bending of the light lets some of it pass through the polarizing filter, producing the colors that you see. Birefringence can be used to identify minerals, using a polarizing microscope. Some man-made materials, such as cellophane tape, also have birefringence. This is one of the science exhibits at Technorama in Winterthur, Switzerland.