Photochromic materials can reversibly change their color and optical properties when irradiated with ultraviolet or visible light. However, they are made from organic compounds that are expensive to synthesize. Fortunately, for the first time, scientists from Ritsumeikan University, Japan, have discovered fast-switching photochromism in an inexpensive inorganic material: copper-doped zinc sulfide nanocrystals. Their results pave the way for a plethora of potential applications ranging from smart adaptive windows and sunglasses to anti-counterfeiting agents.