In explaining the development of children, Piaget believes that there discrete, qualitatively distinct and universal stages. There is a number of characteristics of Piaget's concept of stage.
First, Piaget views the course of intellectual development as progressive changes of cognitive structures. This means children will acquire cognitive skills along their development. For example, in the sensorimotor stage, an infant develops from a reflexive organism to an active body which begins to act intentionally, and at the end of this stage it has object permanence, trying to look for an object which has just disappeared form its sight. Then, children may even develop different conservation skills and at last think abstractly in the formal operational stage. Children's way of thinking determines their intellectual development.
Next, all children go through the same stages in the same order. Piaget divided cognitive development into four main stages, which means the sensorimotor stage, the preoperational stage, the concrete operational stage and the formal operational stage. All children develop from stage to stage in order without skipping one. However, children may not go through the stages at the same time. For instance, some children, who are frequently exposed to experience and education, will develop into formal operational stage earlier than others, because they have proceed more skills cognitively.
Besides, the skills learned in earlier stages are very essential for the development of cognitive skills in later stages. To illustrate, we present two balls of clay of the same size to a child of concrete operational stage. The children can tell that they are the same even if one of the clay balls is rolled into a long shape. This proves that the children develop conservation skills. We have to note that in this case the children also require the skill of object identity...