This paper is issued from a qualitative exploratory study preceding a larger research project aiming to study the implementation of modelling objectives in secondary schools in Geneva. In this paper, we propose to analyse how teachers manage the phase of horizontal mathematization when implementing modelling activities. The aim is to identify and characterize the conditions that foster the transformation of a real-life situation into a problem treatable mathematically. We propose a qualitative analysis of the modalities of the implementation of a well-known modelling task (The Giant's Shoes) in order to enrich the existing empirical results.