According to Jean Piaget, a child during the concrete operational stage is able to recognize that he/she can classify sets of objects on the basis of different attributes or dimensions (e.g. shape, size, color, etc.) Therefore, he/she has the cognitive flexibility to classify sets of objects in terms of different dimensions (e.g. classify objects based on shape instead of based on size).
Dimensions (attributes) of objects:
Classify objects based on size: large objects, small objects
Classify objects based on shape: triangular objects, square-shaped objects, circular objects, etc.
Classify objects based on color: blue objects, red objects, etc.
See:
http://books.google.com.tw/books?id=wYplHc_aPbcC&pg=PT267&lp...