Despite steady research advances in many aspects of virtual reality, building and testing virtual worlds remains to be a very difficult process. Most virtual environments are still implemented using traditional programming language tools like compilers and debuggers. What makes virtual environment construction more difficult is that, on top of having to tackle with the traditional computational and logical errors, it is also an exploration task. Developers must find the right combination of various types of constituents of the virtual environment like the objects, display and simulation details, interaction and modalities, etc. Unlike ordinary programming tasks, for VR, the execution and development environments are different, not just in the temporal sense, but also in the physical sense. We propose that an interactive kernel approach can partly solve this problem and promote higher efficiency in VR content development. Developers can try out and explore different configurations of the constituents of the virtual environment and immediately see their impact (within the virtual world in construction) and optimize the content along the dimensions of performance, interaction usability, realism, and presence. The interactive kernel is part of "PiVOT-2 (POSTECH Virtual reality system development Tool 2)," a collection of tools that support a VR system development methodology called the "CLEVR (Concurrent and LEvel by Level Development of VR System)," previously proposed by the authors [Seo and Kim 2002]. CLEVR/PiVOT-2 is based on a flexible virtual object model called "POM (PiVOT Object Model)," that clearly segregates the aspects of form, function and behavior and the interactive kernel allows the developers to dynamically configure the respective attributes within the virtual world in construction. In this paper, we describe the details of how POM was designed, and demonstrate the effectiveness of using the interactive kernel in constructing virtual worlds.