Photochromic ink is a special type of ink that changes color when exposed to different levels of light. It contains compounds that react to ultraviolet (UV) light, allowing the ink to darken or lighten based on the amount of sunlight or artificial light it receives. This unique property makes it useful for various applications, including printing and packaging. When the light source is removed, photochromic ink returns to its original color. This reversible process can be used in products like sunglasses, posters, and clothing, providing both aesthetic appeal and functional benefits, such as UV protection.