Five year old Jenny and Ten year old Bobby are in two very different levels in relation to the theory of Piaget's stages of Cognitive Development. Jenny, unlike Bobby, may have trouble identifying the same amount of fluid in a differently shaped glass as the same amount, whereas Bobby may have no trouble at all. Jenny will tend to be much more egocentric than Bobby, while he has a perception of other's minds. Bobby will be able to achieve some reversibility and other logic while Jenny is still entirely unable to do this. Jenny's perception of categorization is still made up of simple colors, shapes, or letters, while Bobby understands more specific boundaries. Jenny, not as developed as Bobby, is in the Preoperational stage of development, while her older brother is in the Concrete Operational stage, in accordance to Piaget.
Being in the Preoperational Stage of Development, Jenny may have great trouble perceiving that the same amount of milk, for example, poured into differently shaped glasses (one tall and thin, the other short and wide) is indeed the same amount, she would think the taller glass contained more because she can only take into account how tall the glass is, ignoring that it is thinner in diameter. This is called the theory of Conservation, Preoperational Stage children, like Jenny, do not know how to conserve, whereas Bobby can easily see that the same amount of milk is in both glasses. Jenny is in the "Intuitive Phase" of the Preoperational Stage of Development, which means she is beginning to not be as egocentric, and learn logic in some areas, however, she is still quick to focus on one aspect, ignoring the other. Compared to Bobby, Jenny is much more egocentric, she is unable to acknowledge that other people have minds, too. For example, if Jenny was asked if she has a brother, she may answer "yes", then when asked if her brother has a sister, she may reply with "no", this shows that anything she knows is related to her and expressed in her point of view.