How-things-work visualizations use a variety of visual techniques

to depict the operation of complex mechanical

assemblies. We present an automated approach for generating

such visualizations. Starting with a 3D CAD model

of an assembly, we first infer the motions of the individual

parts and the interactions across the parts based on their

geometry and a few user-specified constraints. We then use

this information to generate visualizations that incorporate

motion arrows, frame sequences, and animation to convey

the causal chain of motions and mechanical interactions

across parts. We demonstrate our system on a wide variety

of assemblies.