Biosensors-devices that sense the human body-are increasingly ubiquitous. However, it is unclear how people evaluate the risks associated with their use, in part because it is not well-understood what people believe these sensors can reveal. In this study, participants ranked biosensors by how likely they are to reveal what a person is thinking and feeling. We report quantitative and qualitative results of two survey-based studies, one on Mechanical Turk workers (n=100), and one on participants in a longitudinal self-tracking study (n=100). Our findings imply that, in the absence of information about particular sensing technologies, people rely on existing beliefs about the body to explain what they might reveal. Highlighting mismatches between perceived and actual technical capabilities, we contribute recommendations for designers and users.