This study attempts to characterise what 7th- and 12th-grade students believe they do not know about artefacts and natural objects, as well as the dependence of what is unknown on a knowledge of these objects. The students were asked to make explicit through questioning what they did not know about a sample of objects. The unknowns generated were categorised according to a scheme based on lexical semantics theory. Two of the categories focused specifically on imprecise unknowns expressed through "What is X?" questions, and on unknowns concerning functions. The results showed, firstly, that a lower grade level and lower knowledge of a certain class of objects was associated with more imprecise unknowns. Secondly, unknowns about the functions of artefacts were significantly more frequent than unknowns about the functions of natural objects at any of the grade levels. Overall, the results were consistent with the hypothesis of a correspondence between knowledge and unknowns.