Thirty-four clinical interviews were conducted with Grade 10 students (15–16 years old) who had received four years of physics instruction. The interview's focus was to understand students' responses from their point of view and not solely from the physicist's angle. The results of the study confirm and deepen, on the one hand, findings from other studies concerning students' severe difficulties in learning the energy concept, the particle model, and the distinction between heat and temperature. On the other hand, students' qualitative conceptions in a new area—the second law of thermodynamics—are revealed. For instance, in the case of irreversibility (i.e., the idea that all processes take place by themselves only in one direction), most students came to conclusions similar to those of modern physicists. But their explanations of irreversibility are based on significantly different conceptual frameworks. The results of the study suggest that a mere enlargement of the traditional physics curriculum by the addition of ideas of the second law is not sufficient to familiarize students with these ideas. A totally new teaching approach to heat, temperature, and energy is necessary. In this approach, basic qualitative ideas of the second law should be a central and integral part from the beginning of instruction.