The child in the Stage of Concrete-Operations has acquired the Concept of Conservation.
Concrete Operations Stage (7 - 11 years): One hallmark of this stage is the concept of Conservation. Up to this time, in the Pre-Operational Stage, the parents had it easy with the Kids. For example, if you poured a child a glass of juice and it wasn’t enough to suite him, you simply poured the contents into a narrower but taller glass. This usually satisfied the need for more.
This worked because at the pre-operational stage the child is barely able to abstract one bond, which in my example is the height.
Please switch to the graphic version to see an example of a PRE-OPERATIONAL thought process, as might occur in a two to seven year old child, or some tenured professors (remember, this is not Harvard).