This qualitative case study aims to address the ways that pre-service mathematics teachers (PSTs) used a rope in a daylong research meeting for cognitive development of children's understanding of counting numbers in Tanzanian elementary schools. Three university mathematics pre-service teachers volunteered participating in this study. Collective notes were taken as data to present the ways that PSTs used a rope in the research meeting for children understanding of counting numbers. Data was analyzed using Bruner's (1966) work on cognitive development. The results show that using a rope, it was found that counting numbers (0, 1, 2, ….,9) were formed, represented visually and symbolically, and read. The finding has implications in teaching and learning counting numbers, including teachers can use a rope to teach the concept of counting numbers among children in elementary schools for a relational understanding of the concept.