Sensor-based interactions are increasingly being used in the design of user experiences, ranging from the activation of controls to the delivery of 'context-aware' information in the home. The benefits of doing so include the ability to deliver relevant information to people at appropriate times and to enable 'hands-free' control. A downside, however, is that sensor control often displaces user control, resulting in the user not knowing how to or being able to control aspects of a system. While this can be frustrating in many situations, it provides new opportunities for enhancing or augmenting various kinds of activities, where uncertainty can be exploited to good effect. We describe how we designed an adventure game for young children that incorporated a number of sensor-based interactions. We also present a preliminary conceptual framework intended to help designers and researchers develop novel user experiences using sensor-based interactions. A set of concepts are provided that characterize salient aspects of the user experience involved in sensing together with a discussion of the core properties of sensor technologies.