Students' conceptual understanding of Newton's third law has been the subject of numerous studies. These studies have often pointed out the importance of addressing the concept of interaction in teaching Newtonian mechanics. In this study, teachers were interviewed in order to examine how they understand interaction and use it in their instruction. The results show that teachers have a wide variety of beliefs about interaction. Moreover, by using the Force Concept Inventory test the understanding of Newton's third law by the teachers' students was tested. According to the results, the explicit use of interaction as a guiding principle in mechanics instruction helps students understand Newton's third law.