Passive systems are designs or technologies that operate without the need for active control or external energy input. They rely on natural processes and materials to function effectively. For example, a passive solar heating system uses sunlight to warm a building without mechanical devices, simply by optimizing the building's orientation and insulation. These systems are often more sustainable and cost-effective than active systems, as they require less maintenance and energy. Examples include green roofs that manage stormwater and improve insulation, and natural ventilation techniques that allow fresh air to circulate without fans.