Over the last century human activities have resulted in a rapid accumulation of carbon dioxide (CO[subscript 2]) in the atmosphere, creating severe damages in the environment. This study investigates how the use of a digital tool, a so-called carbon footprint calculator (CFC), instrumental to calculating CO[subscript 2] emissions of human activities, co-determines high school students' ways of reasoning about their footprint in the context of a global online discussion forum. Our aim is two-fold: 1) to show how what is commonly conceived of as acts of thinking and reasoning are grounded in materiality, in artefacts, and how human agency is shaped by the use of symbolic technologies, 2) to illustrate some of the consequences of this perspective in the specific case of learning about the environment. The findings imply that the received values on the CFC mediate tangible access points to something quite abstract that serve as eye-openers, supporting students' reasoning about emissions. Consequently, this tool provides shortcuts between a given behaviour and the emission associated with it, and it may be seen as exercising agency when put to use in a problem-solving situation, moving human reasoning in specific directions. [For the complete volume, "Designs for Experimentation and Inquiry: Approaching Learning and Knowing in Digital Transformation. New Perspectives on Learning and Instruction," see ED661573.]