Jean Piaget's developmental theory of cognitive processing argues that the process of biological development includes a development in mental processes, since the brain develops with age.
Among the basic tenets of Piagetian theory, it is stated that throughout development we will build schema, or essential knowledge, that will be the foundation to our analysis and thought processes.
In order to build schema, the information that we process will be filtered as we learn more things. It will also be compartmentalized and stored in a variety of sensory endings within the brain. This is the way in which we make connections and relate one thing to another.
An important part of this compartmentalization process comes as a result of conservation. Conservation is basically the observation one makes that an object will remain the same whether it is changed, switched, painted, or physically altered. An example of conservation is when a baby sees his binky on their chair. When you hide the binky, the baby might think that the binky no longer exists. However, as we mature, we realize that the object still exists, but has been only place elsewhere.
According to Piaget, it is during the concrete operational stage (ages 7-11) where children begin to create the concept of conservation. The child first accepts the basic idea that a hidden object is somewhere else, and not that it has just disappeared. Then, the child begins to conceptualize amounts and, finally, the child applies changes such as weight and size as natural physical variations of the same object.