Q.In the conservation experiment with two transparent glasses (Activity 3.2), why might a Class II child and a Class V child give different answers when asked which glass has more water after it is poured into a tall thin glass in front of them?

Jean Piaget held that children actively construct their own understanding of the world, rather than passively absorbing information from their surroundings; as children grow, they take in new information and adapt their thinking to include new ideas, which gradually improves their understanding of the world. Piaget proposed that a child's mind moves through a sequence of qualitatively distinct stages of thought from infancy through adolescence, and it is the different QUALITY of thinking at each stage — not simply the amount of information a child has picked up — that makes one stage more advanced than the one before it.

During infancy (the sensorimotor stage, roughly 0-2 years), the infant experiences the world directly through looking, hearing, touching, mouthing, and grasping, and essentially lives in the present — what is out of sight is, for a young infant, effectively out of mind, since the infant has not yet developed object permanence (the understanding that objects continue to exist even when they can no longer be seen or heard). This awareness develops gradually across infancy, with infants typically beginning to search for a partially hidden object by around 8 months of age.

During early childhood (the preoperational stage, roughly 2-7 years), the achievement of object permanence lets the child begin using mental symbols to represent objects that are not physically present (drawing a person, tree, or house from memory). A salient feature of this stage is egocentrism — children see the world only from their own point of view, which gives rise to animism, the tendency to attribute life-like qualities to inanimate objects. Between roughly 4 and 7 years, children in what Piaget called the sub-stage of intuitive thought ask constant 'why' questions as they build up an understanding of causes. Preoperational thought is also marked by centration — focusing on just one salient feature of a situation (such as the height of a glass) while ignoring other relevant features (such as its width) — which is why a preoperational child will often insist two identical amounts of liquid are unequal simply because they are held in differently-shaped containers.

During middle and late childhood (the concrete operational stage, roughly 7-11 years), intuitive thought gives way to logical thought. Concrete operations are mental actions that are reversible — allowing a child to mentally undo a transformation, which is what enables a 7- or 8-year-old to correctly recognise, in the classic clay-conservation task, that a ball of clay rolled into a long thin strip still contains the same amount of clay as an unrolled, identical ball, because they can mentally reverse the rolling. Concrete operations allow the child to attend to multiple properties of an object at once rather than centrating on just one, which also reduces the child's egocentrism and makes their thinking more flexible — they can now consider alternative approaches and mentally retrace their own steps. Even so, the concrete operational child still cannot manipulate fully abstract ideas divorced from real, concrete objects (such as algebraic steps, or an imagined line of latitude) — this fully abstract, hypothetical reasoning only becomes possible in the later formal operational stage of adolescence (roughly 11-15 years), when thought becomes genuinely abstract, systematic, and capable of hypothetical-deductive reasoning.