A qualitative study of an interpretative nature is presented of the topics that elementary pre-service teachers (EPTs) choose and the types of questions they propose when designing an experimental activity (ExA) as inquiry, after receiving explicit instruction about it. The participants in the study were 154 EPTs organised into small groups to design an ExA. The data were processed using a rubric designed and applied through processes of inter- and intra-rater analysis. The results showed the instruction they had received to be, in general, effective for their formulation of quality questions that can generate ExA-based inquiry. Questions dealing with relations between variables were the commonest. In their free selection of the topic, the EPTs were most likely to ask high-order questions (i.e. ones that foster inquiry) on a wide variety of physics content. There were very few questions concerning biology, and none on chemistry, the environment, health sciences, and so on. After a discussion of the results, specific actions are proposed to improve future EPT training in the formulation of questions with which to initiate school science inquiry.