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 input 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 much more efficient than traditional simulation. It examines roughly 1/6 as many degrees of freedom because the kinematics module precomputes the configurations where parts collide. 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.