Following the teaching of a unit on mass measurement to 274 children (6 to 8 years of age), each of 13 classroom teachers administered an open-ended assessment task. Children represented their perceived knowledge of mass measurement in response to an "Impress Me" prompt. Drawn and written recordings were complex and multi-dimensional, ranging from drawings and/or descriptions of activities children had undertaken or materials they had used, to the articulation of understandings related to foundational ideas of measure. The complexity of categorising and coding the pictorial responses of children is central to this paper. Interpreting children's drawings is not straightforward and this has implications for classroom teachers. [This paper is the fourth in a symposium of four papers. For the first paper, "Using the Drawing-Telling Approach to Reveal Young Children's Mathematical Knowledge," see ED604537. For the second paper, "Capturing the Mathematical Drawing Process Using a Digital Pen," see ED604538. For the third paper, "The Nature of Young Children's Attitudes towards Mathematics," see ED604539.]