This paper utilizes Toulmin's original framework to analyse over 400 answers given by 11-year-olds to a question on a written mathematics test. The question required children to say whether a given statement is true and give a written explanation. Categorizations of answers are developed from the data and examined, suggesting that many children appeared to understand the mathematics but were not able to give adequate explanations. Findings are also compared with other researchers' findings. In contrast to other studies, a large category of non-valid answers appear mathematical, but are largely restatement of the information the children were given. Although only a minority provided explanations deemed worthy of a mark, further analysis demonstrates greater degrees of comprehension than this suggests. Teaching strategies for building children's expressive and specifying skills are identified.