This study examined laypeople's understanding of a simple dynamic system, expressed in reasoning and strategies used by the subjects, and how it affected performance. Participants were 15 undergraduate psychology students. 4 male and 11 female; median age was 24 years, ranging from 21 to 31 years. The subjects' task was to establish equilibrium in a simple predator-and-prey system. A task analysis was performed to identify the problem structure, the vital aspects of the task, and the ideal strategies to perform the task. The subjects' actual performance was compared to these strategies. The results revealed that, even though the task was structurally simple, it was still difficult. Much of these difficulties seemed to stem from a low ability to apply indirect reasoning and thinking in terms of discrete time steps instead of in terms of continuous time.