Tattoos have long been considered something permanent: what is once applied to the skin usually stays with a person for life. But a group of researchers from the University of Colorado Boulder has shown that this is not necessarily the case. The ink can literally be “turned off” and “turned on” with light. In a recent publication in the journal Matter & Light, scientists described a technique where photochromic dyes, enclosed in tiny plexiglass spheres, act as the pigment. Under ultraviolet light, such a design appears, and under ordinary visible light, it disappears.