In this paper we want to present the tools and a methodology to realize web based simulations according to the didactic model defined for the Virtual Scientific Experiment. The strategy that we suggest intends to satisfy all the requirements for the actors involved in the learning process according to the adopted didactic model. These requirements provide that: the tools for building the simulations have to serve a teacher instead of a programmer of web based simulations; the simulation outputs showed at the learner have to focus his attention on the contents instead of the modalities to interact with the presentation; and last but not least, the simulation must not be static, but it evolves according to the different phases expected from the adopted didactic model. From a practical point of view the results reached by us until now are realized by IWT (Intelligent Web Teacher). IWT is a distance learning platform designed with the precise purpose to lay the foundation for the next e-learning generation. We extended IWT with the instruments required to support the new methodology.