We describe Tinker, a high-level design tool that aids the exploration of the design space in sensor network applications. Tinker is targeted at applications that require real-time assignment of semantic meaning to data, rather than just data storage. Tinker lets users write simple programs, as if they were manipulating individual scalar values, and simulates those computations over continuous streams of sensor data. Tinker does not require (or allow) users to specify details such as routing algorithms or retransmission policies, freeing system designers to rapidly iterate among different broad designs before fleshing out details of the one that looks most promising. We demonstrate Tinker’s use in the design and deployment of ElevatorNet, our distributed sensor application that retrofits buildings with per-floor displays of an elevator’s position, determined using barometric altimetry.