In this study, we analyzed the descriptive knowledge and mental models of the phenomenon of tides manifested by 111 preservice primary teachers. The instrument employed is an open-ended questionnaire, analyzed by means of an approach that explores the descriptions, explanations, and predictions in respect of this phenomenon by our subjects. First, we made a descriptive study of the kinds of ideas applied across different dimensions of analysis, and, subsequently, a cluster analysis was performed to check how those ideas were articulated and modeled. By means of this analysis, we were able to identify the mental models underlying the responses of preservice primary teachers. Furthermore, the results showed that the models they did have were not used when it came to making predictions in local situations. Instead, they employed heuristic rules based on everyday assumptions, not always consistent with tidal cycles of approximately 12 h. However, faced with situations that required thinking on a global scale, they did use their models, normally based on alternative conceptions. Lastly, from the results obtained, we consider some of the possible difficulties that preservice primary teachers may have in learning about the phenomenon of tides, and the implications for future teaching--learning designs aimed at overcoming those difficulties.