We present a practical simulation program for rigid part mechanisms, such as feeders, locks, and brakes. The program performs a kinematic simulation of the behavior produced by part contacts and driving motions along with a dynamical simulation of the behavior produced by gravity, springs, and friction. It describes the behavior in a compact, symbolic format and with a realistic, three-dimensional animation. The program is more efficient and informative than traditional simulators. It examines roughly 1/6 as many degrees of freedom because the kinematics module eliminates the blocked ones. It spends little time on collision detection because the kinematics module precomputes the configurations where parts collide. It covers more mechanisms than do previous model-based simulators, generates fuller behavioral descriptions, and exploits kinematics more fully. It uses a simple model of dynamics that captures the steady-state effect of forces without the conceptual and computational cost of dynamical simulation. We demonstrate that our simulation algorithm captures the workings of most mechanisms by surveying 2500 mechanisms from an engineering encyclopedia.