The behavior of large-scale wireless sensor networks has been shown to be surprisingly complex and difficult to analyze, both by empirical experiment and simulation. In this paper we develop a new analytical model of the behavior of wireless sensor networks, based on a fluid approach, i.e., we represent the sensor network by a continuous fluid entity distributed on the network area. The model accounts for node energy consumption, channel contention, as well as traffic routing; thus, it is well suited for describing the properties of sensor networks and understanding their complex behavior.