This paper presents a preliminary study of three six year-old children's use of functional language when engaging collaboratively on a mathematics task. The analysis is presented as an illustration of young children's authority and agency in mathematics as evidenced in their discourse. Modality, as a function of language, was seen to indicate reasoning as a semantic process that expressed a state of knowledge as the children explored number comparison relationships. It is proposed that the children's use of modality indicated an element of internal authority in arbiting mathematical correctness and that related to the nature of the task.