Introduction
Even though tilting a glass and drinking from it is an everyday action, many children appear to be
unaware that the surface of water stays horizontal regardless of the orientation of its container. Piaget
and Inhelder (1948/1956) showed this with their classic water level task, a paper-and-pencil task that
required children to draw the water level in containers that were presented at different orientations.
They concluded that the concept of horizontality is not mastered until 9 or 10 years of age. However