In most virtual worlds, users and agents have limited interactions with the artifacts populating their environment. They can at most operate and sense them with a set of action whose consequences have been predetermined. We propose to extend their range of interaction by introducing interactive semi-qualitative simulation. This would allow them to interactively change the structure of the artifacts and use the simulator's internal models to reason about their behaviors. Virtual prototyping, virtual construction sets, and training systems are direct applications of this technology. In this paper we develop an architecture supporting this concept and report on our current and future implementations.