Many fraction activities rely on the use of area models for developing partitioning skills. These models, however, are limited in their ability to assist students to visualise a fraction of an object when the whole changes. This article describes a fraction modelling activity that requires the transfer of water from one container to another. The activity provides the opportunity for students to explore the part-whole relationship when the whole changes and respond to and reason about the question: When does 1/2 =1/3 ?