Analogies are often used by teachers in their instruction of the concept of function. The explanatory affordances of these analogies for student learning of the formal definition of function, however, are less known. In a planned research paper, we will present a self-generated ‘child-mum biological relationship’ analogy that has been found popular among a sample of Turkish secondary mathematics teachers to describe function and illustrate the potential of such an analogy to unpack the formal definition of function.