This paper describes the specification of a universal sensor interface (USI) called Liquid, which allows for the collection and representation of sensor readings from a wide range of different sensors. We illustrate how it is possible for Liquid to collect data from a broad spectrum of sensors using a select method of sensor classification and to present this data within a common environment. We explore how this approach can extend itself to include sensors not yet conceived of with relative ease. Finally we explain how the Liquid USI provides developers of ubiquitous systems with a general-purpose toolkit for the development of sensor-based applications.