Consider the figure below, which draws the analogy between store$-$and$-$forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth.

Suppose the caravan has $5$ cars, and that the tollbooth services $($that is$,$ transmits$)$ a car at a rate of one car per $2$ seconds. Once receiving serving a car proceeds to the next tool both, which is $400$ kilometers away at a rate of $100$ kilometers per second.

Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth.