This study has the descriptive aim of showing if and how epistemic procedures typical to mathematical reasoning can be practiced by children when they are in a social situation that supports their individual linguistic and cognitive activity. The present paper consists of a fine-grained analysis confronting argumentative skills and epistemic actions of a group of four students functioning in a Grade 9 mathematics class. The four students were presented with a mathematical problem-situation typical of a one year long experiment whose domain was an introductory course about functions. This activity was typical in the sense that: (i) it demanded inquiry; (ii) students worked in groups; (iii) they had computerized tools at their disposition; (iv) they were invited to discuss their work in a whole class forum. The role of the technological tools as a trigger for the application of argumentative skills is investigated.