Resumen: The first contact that a pre-service teacher has with didactics of mathematics is the notion of natural number, being still in the university classroom and not having started working a real classroom. Therefore, the main objective of this work is to relate the knowledge of a trainee teacher to the different difficulties developed by the children and to evaluate the learning processes in a real environment. The participants were 20 future teachers and 40 (9-year-old) children. During the experience, six letters closely related to the contents of the university subject were exchanged; as well as two socialisation letters; and four videos, including two presentation and two farewell videos. Among the results obtained, we highlight that the participating university students have been able to reinforce the knowledge learned in class through the analysis of the children's resolutions, specifically the following: a) counting, b) resolution of additive-concrete situations, c) resolution of multiplicative-concrete situations, d) resolution of a monetary problem, and e) being aware of the manipulation of Cuisenaire's rods. Idioma: Inglés DOI: 10.30862/jhm.v5i1.240 Año: 2022 Publicado en: Journal of Honai Math 5, 1 (2022), 61-74 ISSN: 2615-2185 Financiación: info:eu-repo/grantAgreement/ES/UZ/PIIDUZ-1-180 Tipo y forma: Artículo (Versión definitiva) Área (Departamento): Área Didáctica Matemática (Dpto. Matemáticas)