The possibility of using freehand sketching as the language for interactive design is a longstanding goal. The ability to sketch a 3D object, predict its performance, and redesign it interactively using physics-based feedback would bring the power of state-of-the-art analysis tools into the critical, early design phase. The enormous potential of sketch-based interfaces is widely recognized, and has been broadly pursued. The practical use of such attempts has remained limited because these interfaces have been primarily 2D, loosing much of the benefit of mainstream 3D analysis potential. In order to become truly 3D, the spatial geometry must be automatically -- and quickly -- reconstructed from a single 2D sketch in near real-time. Once reconstructed, it can be converted into a model for simulation, and the simulation results interpreted back into the sketch. This paper presents a system that performs that reconstructs a 3D object from a freehand sketch, and uses the reconstructed object as the basis for a physical simulation. The system represents a first step towards fully interactive physics-based 3D design.