Observation of fellow educators conducting demonstration lessons is one avenue for teachers to develop sensitivity to noticing students' reasoning. We examined teachers' noticing of children's learning behaviours in one demonstration lesson of the "Mathematical Reasoning Professional Learning Research Program" (MRPLRP). The observations of teachers evident in the audio-taped post-lesson group interviews conducted at one school are reported in this paper. The teachers noticed that the children struggled to employ mathematical language to communicate their reasoning and expressed concern about gaps in children's understanding of key mathematical concepts. The teachers viewed limitations in language and mathematical conceptual understandings as a barrier to effective reasoning.