With the increasing prevalence of mobile devices, computing is being carried out on devices with varied resource constraints and is afflicted by constant changes in run-time contexts. It becomes a requirement for software systems to be able to adapt appropriately to the changing computing environments. Functionality Adaptation — dynamically changing how an application carries out its functionality, has not been well studied in the context of pervasive computing. This research aims to devise a flexible functionality adaptation technique for pervasive environments. The foundation of our system is dynamic component composition — applications are made up of small components, called facets, which are brought in at run-time from the network when needed, executed and discarded after use. Which component is used depends on the run-time execution environment, thus adapting the execution of the task. This paper presents the facet model, its implications, and how it can be realized to build adaptable mobile applications.