In another line of research, Schwartz and Black (1999) used a different approach to assess
adults’ abilities to represent water in tilted containers. Their task required the same basic ability
to represent the surface of water as horizontal as in the Piagetian water level task, but it also required
the ability to transform mental representations and knowledge about the role of specific
stimulus properties such as glass diameter and water level. In this task, adults needed to imagine
that two presented glasses of different diameters were filled to the same level with water. When
asked which glass would spill first if tilted, participants were usually wrong. However, the study
by Schwartz and Black included an additional condition that revealed an important divergence in
performance when action plans were involved. When participants were allowed to manually tilt
each glass until the imaginary water would reach the rim, they correctly tilted a narrow glass farther
than a wide one. This research showed that adults are able to imagine the transformation of
the water inside a container and to simulate the tilting movement with their hands without having
explicit knowledge about the correct answer and how it is affected by glass diameter and
water level.