As curriculum scholars in South Africa express concerns about the failures of the outcomes-based curriculum policy that has been introduced post-apartheid it is of increased importance to try and understand the professional development that is required by teachers for its successful implementation. As a contribution to the development of such understanding, this article presents an illustrative case study of a mathematics lesson to emphasise the significance of adequate subject knowledge for effective curriculum design. By examining a learner's response to the use of a mnemonic to teach the multiplication table of 9 in Grade 1 it explains a teacher's reluctance to engage with this response and clarify the relational understanding that connected addition to multiplication within base 10 as denary or decimal system. The discussion draws primarily on data gathered through classroom observation and a stimulated interview conducted to capture the teacher's account of the pedagogic communication she used during the lesson. It underscores the importance of understanding the reasons for rules or procedures taught if the conceptual development required by the policy is to be achieved. In conclusion, the article argues that learners' scientific awareness and appropriate attitude to a subject cannot be nurtured without teachers being in possession of adequate subject knowledge.