This study aims at characterizing elementary pre-service teachers' conscious lack of knowledge about familiar technical artefacts and its relation to their knowledge about these artefacts. The participants were asked to state what they knew and also what they did not know about a sample of familiar technical artefacts such as a fan or a lock. The results showed a difference between the structure of the student teachers' lack of knowledge of these artefacts and the structure of their actual knowledge. These differences were analyzed in relation to Kroes' (Camb J Econ 34:51-62, 2010. https://bibliotheek.ehb.be:2102/10.1093/cje/bep019) model of technical artefacts. Firstly, the student teachers' conscious lack of knowledge was mainly focused on the artefacts' behavior rather than on their components and materials. Secondly, the participants found the function features of the artefacts unproblematic. Thirdly, unknown features about the origin of the artefacts were more frequently cited than the corresponding knowledge features. Finally, non-perceptual properties of the artefacts and the causal relations in which they or their parts are involved were important components both of the students' knowledge and of the students' unknowns.