The aim of this article was to find the connections between teachers' actions (N = 7) and their 3rd graders' performances (N = 86) when solving an open problem. A lot of information about the teaching sessions was gathered by video recordings and collecting pupils' solutions. Teachers' actions were analysed carefully through the phases of a problem-solving lesson (launch phase, explore phase, and discuss and summarise phase). Pupils' solution levels were categorised and their performances were compared in different teaching groups. Based on our data, teachers' way of teaching varied greatly in every phase. In the launch phase, the way in which the teachers presented the problem and especially how they illustrated the central concepts affected pupils' performances. The teacher's way of guiding and the choice of the equipment was central in the explore phase. However, even though some teachers revealed the central ideas of the solution during the launch and explore phases, this had no effect on the pupils' solutions.