Research suggests a developmental shift from forming categories based on perceptual features, to recognizing deeper characteristics and relationships. One process found to highlight deeper properties is comparison between items. The bulk of the research on comparison, however, has been done with familiar items or familiar relationships. An open question remains: under which conditions will comparison help children attend to the deeper properties of novel objects? In two experiments we explore the effect of comparison in a word learning task and its interaction with prior knowledge, language support, and the perceptual features of the compared items. Our results suggest that comparison only highlights deeper similarities when children are given some support to counteract or reduce the influence of surface level features. These results have implications for how to best teach children depending on the amount of prior knowledge that they bring to the task.