Outside the cognitive psychologist's laboratory, problem solving is an activity that takes place in a rich web of interactions involving people and artefacts. This interactivity is constituted by fine-grained actions-perception cycles, and it allows a reasoner's comprehension of the problem to emerge from a coalition of internal and external resources.Taking an ecological approach to problem solving, this article introduces a qualitative method, Cognitive Event Analysis, for studying the fine-grained interactivity between a problem solving agent and his/her environment. To demonstrate the potential of this method, it is used to study a single subject solving the so-called 17 Animals problem using a material model. The fine-grained procedure allows tracking the solution to a serendipity that was brought about because of the participant's aesthetic considerations and a change in her perceptual figure-ground configuration. While a qualitative single-case method cannot prove specific models of problem solving, it questions prevalent mentalist models, and it generates new hypotheses on insight problem solving, because it allows the researcher to attend to outliers and to variability on a fast and fine-grained between-measurement timescale.