Abstract
The study is part of action research aiming at the description of concept development in mathematics. Building on socio-constructivist theories of learning, we pursue the question "How do pupils get to know that shapes which do not appear congruent can have the same area, by building on one another's knowledge in a classroom situation?".
A qualitative design is used in the study. Its participants are Grade 1 pupils of a Prague primary school.
Through the identification of several episodes, we show how quite an unexpected discovery of one pupil in Grade 1 influenced the knowledge construction process of the above knowledge for the whole class. We describe how the class developed shared knowledge and how the teacher influenced the whole process.