Giving effective instructional explanations requires a special type of pedagogical content knowledge, that is, foreseeing where students could employ different assumptions in building their understanding of a specific content. This paper proposes the importance of rehearsing instructional explanations and fine-tuning the coherence and meaningfulness of instructional explanations before pre-service teachers start practice teaching, where many complex factors such as classroom management could occupy their minds, so leaving little room for focusing and reflecting on their instructional explanations. The current paper presents actual examples of such rehearsals in which the pre-service teachers demonstrated the need for developing deeper pedagogical understanding of the ways their elementary school students construct their content knowledge. It concludes with the suggestion to provide pre-service teachers with ample opportunities actually to practice explaining specific concepts and engage in "post-hoc" pedagogical analyses of their explanations in teacher training programmes. (Contains 3 figures.)