Children’s behavior on Piaget’s conservation-of-liquid-quanity problem is often used to exemplify the idea that development is discontinuous. The child first sees equal amounts of liquid in similarly shaped glasses and an empty, differently shaped glass. Then, the child sees the liquid from one glass poured into the differently shaped glass. Finally, the child is asked whether the amount of liquid remains the same or whether one glass has more. Young children, like this girl, are unshakable in their belief that the glass with the taller liquid column has more liquid. A year or two later, they are equally unshakable in their belief that the amount of liquid in each glass is the same.

ALL: BIANCA MOSCATELLI/WORTH PUBLISHERS