The purpose of this paper is to describe some contingent moments during an in-service primary teacher's lesson on area, to analyse the teacher's responses, and to examine the relationship between the teacher's mathematical knowledge in teaching and her responses to these contingent moments. The teacher, in a public primary school located in Ankara, Turkey, taught five lessons on area measurement. During these lessons, the teacher was faced with contingent moments that she had not expected. Data for these moments were collected in field notes, video, and audio recordings of interviews with her. In this paper, five contingent moments are presented and discussed. In each case, after describing a contingent moment, the teacher's responses were analysed with reference to the codes of the Contingency dimension of the Knowledge Quartet model (Rowland et al., 2005). The analysis revealed that the teacher was able to acknowledge and incorporate her students' ideas into her instruction and respond to the unavailability of materials, and to turn them into opportunities. Although the teacher had difficulty in responding to the contingent moment that was initiated by herself, what was important here was that the teacher did not ignore her insight, instead; she tried to respond.