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.