Each analogy has conceptual advantages and disadvantages with respect to the others. For example, the popular water flow model reinforces a source-receiver model of electricity and implies that electrons move rapidly in the same direction; a bicycle chain model similarly implies that electrons move in the same direction but does not suggest a source-receiver model; and teeming crowds make it possible to conceive of electron flow in an electric circuit as occurring slowly and randomly, albeit drifting in a common direction. The water flow and teeming crowds models offer one representation of resistance (narrowing in the pipe through which the water is flowing or in an opening through which the crowd has to pass), whereas a bicycle chain model is limited in representing resistance