The aim of the study was to explore teacher support to students’ mathematicalreasoning when they retrieve and use prior knowledge to learn new mathematicalcontent. Two lessons were designed, one with the purpose to engage students inretrieving prior knowledge of angles and shares, and the following to learn how toconstruct pie-charts. The teacher’s interactions with students were recorded and theresult showed that the teacher could support students’ reasoning by showing inte-rest in their thinking and asking challenging questions regarding clarification andjustification of their mathematical reasoning.