In his writing, I report on how two 12-year old children used fraction strips to add 1/2 and 2/5. In their interaction with the tool, I look for the emergence of the tool-mediated Zone of Proximal Development to analyse the knowing that become available to the children. In thinking about this interaction, I ask what is the role of the more knowledgeable other? Who/what becomes one, in an interaction? Who possesses what is called "knowledge" more than the other, to then becomes "the more knowledgeable other"? Through the analysis of a transcript expert, I think about and tackle these questions.