In this paper we investigate the effects of the app Fingu on the part-whole understanding of children. Using the Artifact Centric Activity Theory (ACAT) framework, we analyse the externalisation and internalisation processes that appear to occur during one child’s use of the app. Our analysis of a 13-minute video of a child playing the app Fingu indicates that the design of the app affected the child’s experience of part-whole relationships in ways that were likely unintended by the app designers. We found that the child used less sophisticated part-whole strategies as the gameplay unfolded and suggest that, due to the limitations of the use of one video, further research is required into the experiences of other children whilst playing Fingu.