Amother gives her three-year-old and her four-year-old children chocolate milk in identically shaped glasses to avoid conflict over who has more. the mother is showing her awareness of her children's difficulty with:

The answer to this question is Conservation.

Conservation in psychology means the ability to know that even you change the size or shape of a container the mixture inside it still have the same volume. Conservation is the theory of the psychologist Jean Piaget. The lack of conservation in kids is always been observed when kids chooses the tallest glass of juice because they thought that it has more juice even if the glass is the thinnest from the glasses.