There is a great potential opportunity to use portable physical activity monitoring devices as data collection tools for educational purposes. Using one such device, we designed and implemented a weeklong workshop with high school students to test the utility of such technology. During that intervention, students performed data investigations of physical activity that culminated in the design and implementation of their own studies. In this paper, we explore some of the mathematical thinking that took place through a series of vignettes of a pair of students engaged in analyzing some of their own activity data. A personal connection to the data appeared to aid these students in recognizing their own errors, and ultimately helped them move from a point-based analytical approach for making sense of the data to an aggregate one. From our observations of this designed learning experience, we conclude that physical activity data recording devices can afford students the opportunity to reason with personally relevant data in meaningful ways.