This paper reports on an analysis of student conceptions in responses to explanation-type questions in a grade 12 (17-18 years) South African national physics examination. We were guided in our analysis of 921 examination scripts by a framework developed by Dagher and Cossman on types of explanations in science which enabled us to understand the characteristics of the explanation, and then diagnose a naive conception that was inherent to the explanation. We sought clarity on the naive conceptions by interviewing 10 students on their responses to these questions. The study revealed that students have a naive, superficial and fragmentary understanding of scientific phenomena. These conceptions were not theoretically grounded as students appeared to string together pieces of knowledge in response to contextual feature of the problem situation. This supports the interpretation of the student conceptions from a p-prim perspective as advanced by diSessa and his colleagues, and challenges the notion of students having misconceptions. We believe the method of analyzing the explanations does offer researchers and teachers a reliable and efficient way by which written student explanations can be probed for conceptions.