A demonstration model is a model that shows how a plan can be achieved through the use of physical objects. Emmett Brown built two demonstration models to illustrate to Marty McFly how to get the DeLorean time machine to travel through time. However, Doc would always regret on each occasion about the crudity of the model as well as not having had time to either paint it or get it to scale; fortunately, Marty always reassured him that the model looked fine.
Doc's 1885 demonstration model
The 1885 model was built in Doc's stables on September 4 and September 5 of that year, mostly out of wood parts. It featured the main railroad line which spurred off into Carson Spur. Carson Spur terminated three miles down the track at Shonash Ravine. Other landmarks that Doc added included the silver mine where the DeLorean would be loaded onto the tracks and the locomotive would begin to push it. The Point of No Return was represented by a windmill, indicating it would be too late to stop the train before the edge of the ravine.
Doc and Marty ran the experiment with the model train on one side. It was powered by a form of electric motor that Doc hooked up to the tracks, which passed current through them. They both watched as the train came around the bend, hooked onto the spur, and pushed the DeLorean to the edge. Doc caught the model DeLorean just as it drove off into the 'ravine'. Soon thereafter, Clara was at the door of the stable.
Doc's 1955 demonstration model
The 1955 model was built by Doc's younger self in his garage on November 7 from paper, cardboard boxes, metal containers, bottles, and salt and pepper shakers all painted to a gray color. The model was surprisingly accurate in representing Courthouse Square.
Doc gave Marty a toy car that had to be wound up to travel down the road. Doc hooked up to a power source and pointed an alligator clip to the pole on the model of the Hill Valley Courthouse. He instructed Marty to release the car, and timed the 'lightning' with its intersection of the cable. The result was an explosion on the model table, with the car in flames. The model car drove off the table and towards a large cloth, setting it on fire. Doc gasped in panic and ran to put out the fire with an extinguisher. Soon thereafter, Lorraine was at their door.