This article compares two Interactive Reading sessions between a teacher and 4- year old students, at two different periods of the school year (beginning and end). The activity consisted of "reading" a picture book together, the first instance in a broader didactic sequence of emerging reading/writing skills. The two sessions were analyzed according to a comprehensive-interpretative procedure used in the qualitative analysis of the complex processes inherent to didactic microgeneses. These focus on two subjects of study: the progression of teaching/learning content, here in an Interactive Reading situation, and the progression of the understanding modalities underlying verbal interactions. Our hypothesis is that positioned learning can only be effective when there is co-construction of content specifically designated for teaching, this co-construction itself dependent on that of a common zone of understanding between the partners. Between the beginning and the end of the school year, it was shown that not only did the teaching/learning content become more complex, but that the zone of understanding created while reading the book also changed. Finally, the students' progress in psycholinguistic skills over the course of the year made it possible to identify the knowledge that they have interiorized from the dynamic of classroom interactions.