Skin conductance is an interesting measure of arousal level, largely unfamiliar to most end-users. We designed a mobile application mirroring end-users’ skin conductance in evocative visualizations, purposefully made ambiguous to invite rich interpretations. Twenty-three participants used the system for a month. Through the lens of a practice-based analysis of weekly interviews and the logged data, several quite different—sometimes even mutually exclusive—interpretations or proto-practices arose: as stress management; sports performance; emotion tracking; general life logging; personality representation; or behavior change practices. This suggests the value of a purposefully open initial design to allow for the emergence of broader proto-practices to be followed by a second step of tailored design for each identified goal to facilitate the transition from proto-practice to practice. We contribute to the HCI discourse on ambiguity in design, arguing for balancing openness and ambiguity with scaffolding to better support the emergence of practices around biodata.