The role of scientific reasoning in promoting scientific literacy during the early years of schooling is well acknowledged. Using the analytical framework for scientific reasoning skills by A. Opitz et al. (2017, "Educational Research and Evaluation," 23(3-4), 78-101), this multi-age observational study investigated the extent of scientific reasoning among early graders and the instructional design features that influence their reasoning processes. A group of three early grade learners and one fifth grader was assigned to solve an ill-structured problem involving rolling symmetrical and asymmetrical cups down an inclined plane. The activity lasted for almost 2 hours and was videotaped and subjected to an analytical framework. The results indicated that the children could reason scientifically on aspects such as hypothesis generation, evidence generation and evidence evaluation. However, the children did not perform well in aspects such as questioning and problem identification. Instructional design features such as the researcher's probing, collaborative problem solving, heterogeneous age groups, using a familiar language and the nature of the problem were found to have varying influence on children's reasoning processes. The study documents spontaneous scientific reasoning for early graders which can serve as a building block for developing scientific reasoning skills in later years, in particular designing relevant learning materials and teaching strategies. Implications for practice and further research are discussed.