Photochromic inks are special types of inks that change color when exposed to different levels of light. They contain compounds that react to ultraviolet (UV) light, allowing them to darken in sunlight and return to their original color in the absence of UV light. This unique property makes them useful for various applications, including printing on packaging, labels, and textiles. These inks are often used in products like sunglasses and smartphone cases to provide visual effects and enhance user experience. The technology behind photochromic inks is based on reversible chemical reactions, making them a fascinating area of study in materials science.