In this study, we use children's prior knowledge to support their development of cardinality understanding, based on Bermejo's (1996) model of cardinality understanding and on the distinction between cardinality (the number of items in a set is represented by a number-word) and the cardinality principle (the last number-word used in counting determines the cardinal number of a set). Two groups of 24 children each were selected by means of a pretest. The experimental group completed a learning program centered on cognitive conflict, whereas the control group did not receive any special intervention. Results show that most of children in the experimental group reached full understanding of cardinality. In addition, we provide evidence that cardinality is not a component of counting, but that counting and cardinality are different things, just as forms and functions or means and goal are different; and that it is number which gives meaning to subitizing and counting.