This study examined how young children change their strategies for comparing sizes. 33 5-and 6-yr.-old children were asked to compare the sizes of geometric figures, and their strategies were observed. Children in experimental conditions were also asked to evaluate presented strategies for comparing geometric figures, and half of the children were provided with feedback on the evaluation. Analysis showed children provided with feedback on evaluation showed increased use of the strategy of placing one figure on another. from Nissan Science Foundation and by Grant-in-Aid for Scientific Research C, 14510147.