Physical phenomenon can be expressed in multiple forms like equations, tables, graphics and animations that respond to underlying logic. With this idea in mind, applications have been constructed that allow students and secondary and university teachers to utilize mathematics to create or to explore interactive models.
MODELLUS is a tool that allows making multiple representations (tables, equations, graphics, animations) of studied phenomenon, with manipulation of variables in concrete or abstract forms. It is distributed almost for free (it costs $5 to receive the book and CD in the mail). It is supported in several languages through which examples can be analyzed and information can be gathered concerning the function of the system. Modellus can be learned experientially and through inquiry; online help is also available. One can join the community of virtual Modellus users in Yahoo Groups in order to share experiences and to search for solutions to problems that surface when using the system.