Several physical quantities are unchanged, or conserved in the face of spatial or configurational transformations. As Piaget noted, children in the early preoperational period fail on all of these tasks, typically giving answers that conform to the most salient dimension (e.g., in the number conservation task, 3- and 4-year-olds typically state that the longer transformed line has 'more'). Children in the late preoperational period often succeed at some of these tasks, but fail to provide adequate justifications for their judgments. It is not until the concrete operational period that children can reliably supply logical justifications, such as reversibility, for conservation.
The seven tasks are not acquired
at once. instead, they are acquired in the order listed here, with
conservation of number typically mastered by 5 or 6, but conservation of
volume often not mastered until 9 or 10. This was something of an
embarrassment for Piaget, who invoked the notion of decalage ('uncoupling')
in an attempt to explain why such structurally similar concepts should
be acquired at such diverse ages.