Nanopaint is a coating that can modify the properties of a surface or substance according to user-defined parameters. Like ordinary paint, nanopaint is applied as a liquid and then hardens. The liquid contains a suspension of microscopic particles called nanotube s that alter their behavior as external conditions change or when a specific command is given. Nanopaint is in the research-and-development phase. Engineers have produced a prototype nanopaint that can block RF (radio frequency) fields in much the same way as a metal, such as copper, can do. When applied to the interior walls of a building, the material can selectively pass or impede signals to and from cell phones, portable radios or other wireless devices. Potential applications of nanopaint abound. One especially interesting idea is the use of nanopaint on the exteriors of buildings to alter their infrared (IR) reflecting or absorbing properties depending on external conditions. This could improve energy efficiency by helping structures absorb thermal energy on cool but sunny days, reflect it on hot days, retain thermal energy on cold nights and radiate it away on warm nights. Specialized nanopaints might perform an almost endless number of other functions, such as: (1) Block cell phone signals in inappropriate environments, such as theaters, hospitals and funerals. (2) Give glass the ability to become more or less opaque as desired. (3) Give the surfaces of motor vehicles or industrial machines the ability to repair themselves when damaged. (4) Allow the textures of surfaces to be altered at will. (5) Discourage the growth of pathogens such as bacteria or viruses. (6) Repel or neutralize toxic chemicals, acids or other corrosive agents