The past century has seen a debate on what characterises a scientifically literate citizen. Originally, scientific literacy implied that a citizen should know the products of science but has grown to incorporate processes of science and aspects of the nature of science. Studies on students' epistemic knowledge are rarer than ones on students' content knowledge. However, PISA has made an innovative attempt to measure epistemic knowledge. The purpose of this study is to investigate qualitative features in student responses to epistemic items in PISA. Six constructed response items on epistemic knowledge from PISA 2018 Science were chosen. The student responses coded as 'full credit' were put aside and thematic analysis was used to analyse the remaining 'no credit' responses. The results show that it is common to focus on the scientific phenomenon (content knowledge) or the method itself (procedural knowledge) instead of epistemic knowledge. One feasible explanation is that epistemic knowledge is intellectually demanding; another is that epistemic knowledge is still overshadowed firstly by content knowledge and secondly by procedural knowledge. The fact that students tackle epistemic aspects of science as content or procedural knowledge suggests that teaching should more explicitly distinguish between these different aspects of scientific knowledge.