Since the viscous behaviour of liquids is evident in many everyday situations, it is plausible to assume that students have (pre-)conceptions about the physics behind these phenomena. As yet, no reliable studies exist on this topic. Therefore, the present study aims to address this research gap by exploring secondary school students' conceptions about the viscous behaviour of liquids. For this purpose, we conducted structured interviews with 12 ninth grade students, in which the students were asked to describe and explain various viscosity phenomena of everyday life, presented to them as video clips. Our analysis of the interviews revealed that many of the students have the conception that density is associated with viscous behaviour. In addition, some students explain viscous behaviour with reference to the ingredients of a liquid or by its stickiness or compression. Further, both macroscopic- and sub-microscopic-level explanations for the viscous behaviour of liquids emerged in the interviews.