This study was conducted for three purposes. First, we assessed the development of children’s concepts of hardness as an intensive physical quantity. We hypothesized a U-shaped behavioral growth curve for children’s judgments on tasks measuring intensive physical quantity, but the hypothesis was not confirmed. The justifications accompanying correct and incorrect judgments for hardness were similar to those where U-shaped behavioral growth was found for other intensive physical quantities (temperature and sweetness). The second purpose was to determine how children solve seriation and transitivity tasks where hardness was the content. More older than younger children solved these tasks, and the transitivity task was correctly solved after the seriation task was correctly solved. One pattern of solutions for transitivity and intensive physical quantity allowed for differentiation between younger and older children who solved the intensivity tasks correctly. The third purpose was to determine whether children think that objects’ hardnesses are labile (i.e., hard objects can become soft and vice versa) by simply adding more of that substance. More younger than older children thought that this was true.