In this article I describe an approach to task design and implementation that addresses the broad capabilities common to 21st Century Skills and the STEM education agenda. Principles of task design and implementation were co-constructed by ten teachers and a single researcher during a longitudinal study that took place over 3 years. A vignette that draws on observation data from a Year 1 class and a post-lesson teacher interview is used to illustrate the possibilities that exist for mathematics to support critical reasoning and enquiry in primary science. The article concludes with a reflection on the role of teachers in designing and implementing tasks aimed at promoting effective STEM teaching and learning.