Heat-activated inks are special types of inks that change color or become visible when exposed to heat. These inks are often used in various applications, such as security printing, packaging, and novelty items. When heat is applied, the ink reacts, allowing for hidden messages or designs to appear. These inks can be found in products like thermochromic materials and heat-sensitive labels. They are popular for their ability to provide an interactive experience, as users can reveal information by simply applying heat, such as rubbing or using a heat source.