<P> Conservation refers to a logical thinking ability which, according to the psychologist Jean Piaget, is not present in children during the preoperational stage of their development at ages 2--7, but develops in the concrete operational stage at ages 7--11 . Conservation refers to the ability to determine that a certain quantity will remain the same despite adjustment of the container, shape, or apparent size . </P> <P> Conservation tasks test a child's ability to see that some properties are conserved or invariant after an object undergoes physical transformation . The following tasks also explain the different types of conservation . Piaget proposed that children's inability to conserve is due to weakness in the way children think during the preoperational stage (ages 2--6). This stage of cognitive development is characterized by children focusing on a single, salient dimension of height or length, while ignoring other important dimensions about a situation . Children during this stage also tend to focus on the static characteristics of objects, instead of focusing on when objects undergo changes, which is a critical element of the following tasks . </P>

In which stage of cognitive development does the conservation concept first appear