In this article, the author focuses on possible ways to develop the concepts of joint variation and function through the upper primary and lower secondary years of education. The examples demonstrate the building of a range of important mathematical ideas by modelling life-related situations using students' informal and intuitive knowledge. Over several years, ideas such as dependency, continuity, gradient, and function can be gradually formalised. Students are modelling life-related situations with functional relationships and representing the relationships in a variety of forms--tables, graphs, equations--and in everyday language. As students work with more complex functions, the need for syntactic manipulations of terms in symbolic equations arises and can be introduced in the context of the situation being modelled. Students can also be introduced to the use of technology in the forms of graphics calculators, CAS calculators, and spreadsheets as they work with functional models.