Rochelle has a glass of Kool-Aid. She pours her Kool-Aid into a toy teacup, and then she pours the Kool-Aid from the teacup into a beer stein. She then pours it from the beer stein back into the original glass. She knows the amount of Kool-Aid has not substantially changed. What does this exemplify?
a. abstract thinking
b. abstraction
c. preoperational stage
d. reversibility

Jean Piaget proposed, as part of his cognitive development theory, that around ages 7 to 11, children develop the idea of "conservation" which is paired with the concept of "reversibility"

Conservation refers to the thinking ability that allows a person to know that a certain quantity of liquid or mass will remain the same despite being changed of container, shape, or apparent size. This means, that actions can be reversed (reversibility)

In this example, Rochelle has a glass of Kool-Aid, she pours it into a different container (a teacup) and then into a beer stein and then into the original glass. Younger children would think that the teacup has less liquid (since it's smaller) and that a significant amount of liquid is lost while doing all this changes from recipient to recipient.

However, Rochelle knows that this quantity of Kool-Aid remained the same despite being changed of container and that after being changed from container to container and then back to the first one, the action was reversed and went back to its original point.

Thus, this is an example of reversibility.