In this article, we present kindergarten children's ideas about thermal phenomena before any educational intervention took place. In order to capture and account for the heterogeneity of the kindergarten group in this study, first teachers observed children's exploration behavior, task orientation, science interest, and language comprehension in everyday kindergarten life using a structured observation form. Then, 24 children aged between 3.8 and 6.0 years were interviewed individually about three situations focusing upon water temperature and its changes. The results show that interest in science and language comprehension are significantly related to children's understanding of thermal phenomena, while task orientation and exploratory behavior are not. In general, the kindergarten children did not yet use the word "heat" in their descriptions and explanations but were more or less able to describe the water temperature and its changes in a differentiated way.