The purpose of this study has been to investigate how the activity with causal linking words may contribute to developing pupils' oral reasoning skills in small groups. The empirical data has been collected through two qualitative research methods: open but passive observations and group interviews. The result has been analysed based on the presented theories and previous research. To investigate this purpose, two questions were formulated:  How can working with causal terms in combination with small group discussions help to develop pupils' oral reasoning skill? What is the significance of the interaction between the pupils for the way they work?  The activity used in the intervention, where the pupils had to put sentences together by placing concept cards and arrows, takes off in relevant previous research and the concept of collaborative support.  The result of the study indicates that the pupils feel that their causal reasoning is positively affected, by using linking words to develop their reasoning in small groups. However, there is still a need for pupils to learn about the meaning of linking words, and how to use them in the correct context.