The use of original sources is a useful resource not only to be used with secondary school students but also with prospective mathematics teachers. In this work, we designed a series of tasks based on a fragment excerpted from Clairaut's "Éléments de Géométrie" to be carried out with 24 participants enrolled on a Masters' Degree in Secondary School Mathematics Teaching. This fragment was chosen both due to its content and to its narrative structure and our main goal was to determine which elements of professional knowledge were used by prospective secondary mathematics teachers when reading this fragment. In order to do so, we used the MKT model as an analytical tool and we also assessed some aspects related to literacy skills. The prospective teachers were able to recognize mathematical and pedagogical components within the source that relate to their future practice. In addition, the participant's literacy skills seem to play a role in the richness of their reading.