1. A method of conducting operator-assisted machine simulation to explore the effects of different rules used in a modular manufacturing or assembly system, said method comprising the steps of: (a) providing a computer, a serial hub, and a plurality of physical mock machines set apart from the computer, networked through the serial hub to the computer, for use in simulating operations performed by machines in the manufacturing or assembly system, each mock machine including a simulating device for representing a different machine in said system, said simulating device including controls to allow the participation of an operator in real-time simulation modeling of said system, wherein said controls represent a plurality of generic stages of operation of each said machine and include at least one of physical buttons, lights, switches, slides, and levers for simulating at least a loading stage, a processing stage, and an unloading stage of said machine operation and for allowing the operator to manually interface therewith in response to conditions necessitating operator intervention during said simulated loading stage, processing stage, and unloading stage; (b) providing at least one tangible block representing a part being processed by said machines in said system, said one tangible block adapted to be manually placed in one of said mock machines by the operator and moved by the operator from said one of said mock machines to another one of said mock machines during the simulation; (c) constructing a first set of simulation modeling rules for defining elements of said system, and relationships among said elements, by the operator; (d) laying out said simulating device and said tangible block in accordance with said system to be simulated, by the operator; (e) providing an interface to allow the operator to interface with the computer to define a second set of simulation modeling rules for each said simulating device; (f) defining a third set of simulation modeling rules for the operator to follow when simulating said system, by the operator; (g) enabling the operator to select one of said mock machines for initiating the simulation of the operation of the corresponding machine of the manufacturing or assembly system, said one mock machine signaling the operator when the simulated operation is completed, enabling the operator to select a further one of said mock machines to simulate a further operation of said manufacturing or assembly process during the simulation; (h) executing operations on each said simulating device according to said first, second, and third sets of simulation modeling rules and allowing said operations to proceed to completion without modification of said first, second, and third sets of simulation modeling rules; (i) monitoring, by the operator, the operations performed by each said simulating device; and (j) using the monitoring results to produce simulation results.