Obesity is a critical global public health issue. It leads to numerous health conditions that limit the quality and length of life, placing a huge burden on healthcare systems around the world. Scholars believe that the fight against obesity can be won only through multidisciplinary efforts. However, the literature on people's understanding of obesity is limited, perhaps because this topic does not attract the attention of educators at the appropriate level. Although existing studies show that people lack understanding about the mechanisms of obesity, most of the arguments in these papers suffer from a lack of theoretical background and methodological problems. Therefore, we conceived the present study based on a sound theory (conceptual change) in order to understand school students' conceptual patterns regarding weight gain, which is an important component of obesity knowledge. A written questionnaire probing the mechanism of weight gain was distributed to 151 secondary school students in the Turkish part of Cyprus. Content analysis was applied to students' compositions. The results showed that students have six major conceptual patterns about weight gain. They also hold a range of misconceptions and misunderstandings, and sometimes have no conceptions. At the end of the paper, we discuss possible curricular and teaching implications based on the patterns and incorrect knowledge structures.