Material can be moved through a process by pushing it or by pulling it. A chain provides a good, visual illustration of pull versus push. If a chain is stretched out on the ground and then pushed from one end, the links of the chain tend to pile up in a heap instead of moving smoothly forward. However, if the chain is stretched out and pulled from one end rather than pushed, the chain moves forward very easily and there is no piling up of the links. In the push case, the links piling up are analogous to inventory building up in the flow of material.
