This paper considers the role of comparison in the development of knowledge. Results show that comparing similar objects makes them appear more similar. Comparing dissimilar objects, on the other hand does not make them appear more similar, and in some circumstances may make them appear less similar. The effect of comparison on similar items was especially striking since participants judged items to be more similar after comparison even if the comparison task was to list differences between the two items. Further, this effect appears specific to comparison and does not appear to be simply due to a "fleshing out" of object representations (listing properties of two objects without comparing the objects themselves served to increase the objects' similarity regardless of whether the objects were similar or dissimilar to start). This suggests that comparison may play a special role in partitioning bits of experience into categories, sharpening categorical boundaries, and otherwise helping us create conceptual structure above and beyond that offered by the world.