This article aims to contribute to the theory of mathematics instruction by highlighting and analysing Leen Streefland's work as a teacher in a primary school. Core ideas of Streefland's are discussed as part of a recent wave of educational innovations using the idea of learning communities. I present a case study of a lesson co-taught by Streefland and a primary school teacher (students between 11 and 13 years of age). Streefland addressed the students as "researchers" and gave them realistic problems to work on. Since they provided occasions for increasingly sophisticated solutions, the tasks given to the students stimulated processes of vertical mathematization. The students' role as researchers required them to construct novel ideas and present these to their classmates. The various solutions proposed by the students were used by the teachers to structure the learning activities in the classroom. The interaction between the common understandings in the classroom and the learning process of individual students shows that the classroom activities amount to a process of collective reflection.