An active structure (also known as a smart or adaptive structure) is a mechanical structure with the ability to alter its configuration, form or properties in response to changes in the environment. The term active structure also refers to structures that, unlike traditional engineering structures (e.g., bridges, buildings), require constant motion and hence power input to remain stable. The advantage of active structures is that they can be far more massive than a traditional static structure: an example would be a space fountain, a building that reaches into orbit.