When pre-operational children are shown two equal-size glasses filled with equal amounts of liquid, they correctly say that neither glass “has more.” But when the contents of one glass are poured into a taller, thinner glass, they incorrectly say that the taller glass now “has more.” Concrete operational children do not make this mistake because they recognize that operations such as pouring change the appearance of the liquid but not its actual volume.
BIANCA MOSCATELLI/WORTH PUBLISHERS