Open-ended investigations give pupils the opportunity to work creatively to solve a problem. But what ideas need to be taught to enable such creativity? The empirical work reported in this case study shows that an understanding of substantive ideas is not sufficient. A procedural understanding is important. Explicit teaching of the concepts of evidence, and particularly ideas associated with uncertainty in data sets, improves pupils' investigations. The proportion of pupils who then work creatively, responding iteratively throughout the investigation, increases. The curriculum implications are discussed.