In this paper, we respond to a research study from the last CERME conference and offer an alternative study collecting data via a Concept Cartoon. Our qualitative study investigates how future primary school teachers respond to the Concept Cartoon that is based on a task possibly triggering natural number bias. The task is situated in the domain of fractions and seeks numbers that lie in the middle between given two fractions. We accompany it with a fragmented set of questions that focus on respondents’ solutions as well as on their assessment of the solutions and comments of hypothetical pupils presented in the cartoon. According to the results, this instrument of data collection proved its usefulness in revealing natural number bias as well as other knowledge deficiencies related to the topic of fractions or to the learning of mathematics in general.