Theory Jean Piaget

A child in the preoperational stage also lacks the principle of conservation. This is the knowledge that quantity is unrelated to the arrangement and physical appearance of objects. Children who have not passed this stage do not know that the amount, volume or length of an object does not change length when the shape of the configuration is changed.  Piaget declared that this is not mastered until the next stage of development. The concrete operational stage lasts from the age of seven to twelve years of age. The beginning of this stage is marked by the mastery of the principal of conservation.