When a material detects a stimulus in its environment, it can respond in a useful, reliable, reproducible, and reversible manner. These properties are useful in a variety of fields, including dentistry. Materials used in dentistry were designed to be passive and inert, interacting with body tissues and fluids as little as possible. The ability of materials used in the oral cavity to survive without interacting with the oral environment was frequently used to evaluate them. The realisation of the benefit of fluoride release from materials was the first inkling that a "active" rather than "passive" material might be appealing in dentistry.