We take the unorthodox view that the operating system ought to shoulder responsibility for converting raw sensor data to Contextual Data Units (CDUs). The CDU is simply an abstract data type that embodies a unit of context meaningful to applications. A concrete CDU such as user motion state might take values such as sitting, standing, walking, and driving. In contrast, traditional OSs offer apps opaque bytes, a very low level and unassuming interface to data. We suggest that the right approach for mobile systems is a much higher level of data abstraction: the CDU.