This research investigates teacher activities and the improvement of children's cognitive development through bottle cap media and uses a Collaborative Classroom Action Research (CAR) approach with class teachers. The data collection was conducted through observation and analysed descriptively and qualitatively. The research subjects involved children in Group A, consisting of 9 participants. The first cycle results show that teacher activities received a score of 88% with good criteria, and children's cognitive development reached 44% with Developing as Expected (BSH) criteria. Meanwhile, in the second cycle, teacher activities improved with an average score of 99%, categorized as very good. Children's cognitive development also increased to 88%, and they were classified as Developed Very Well (BSB). Based on the research findings, bottle cap media enhances children's cognitive development.