This paper aims to evaluate a teaching methodology for the learning of scientific concepts in a primary school context. The focus is on the role played by individual writing, associated to a collective observation of an experiment and a classroom discussion. The hypothesis is that this methodology shows its effect both, on the scientific quality of written descriptions of experiments and on children's metacognitive thinking. The participants were 172 primary school students, attending grade III, IV, and V. For each grade, two groups were formed and both observed an experiment. The experimental group's students wrote individually what was observed in the experiment, discussed it in the classroom, and wrote again individually a report on what was observed and discussed. The control group's students discussed what was observed in the experiment and wrote individually a report on what was observed and discussed. Different experiments have been carried out depending on the grade (combustion, evaporation, and conservation of liquids). The results substantially confirmed a better effectiveness for the pattern "observation-individual writing-discussion-individual writing". Moreover, the experimental groups' students showed a more metacognitive thinking than the control groups' ones did.