The application to rational numbers of the procedures and intuitions proper of natural numbers is known as Natural Number Bias. Research on the cognitive foundations of this bias suggests that it stems not from a lack of understanding of rational numbers, but from the way the human mind represents them. In this work, we presented a fraction comparison questionnaire to 502 school children from 5th to 7th grade to investigate if the Natural Number Bias succeeds in explaining their error patterns. About 25% of children responded in a way perfectly consistent with the Bias, but good students committed many errors in items that the Bias predicts to be easy. We propose an explanation based on comparison strategies and wrong generalizations of a common remark used for teaching fraction magnitude. [For the complete proceedings, see ED597799.]