"In one version of the Piaget and Inhelder (1956) test, the subject is shown a bottle partially filled with water and is told to notice the way the water fills the bottle. The subject is then asked to predict where the water will be when the bottle is tipped. Piaget and Inhelder believed that the relevant knowledge about the horizontal would be attained at an average age of 10 years. The Water-Level Test as originally conceptualized by the developmental psychologist Piaget was never intended to test anything about water per se. It was meant to be a task of spatial concepts, in this case the ability to use a Cartesian coordinate system to represent space—but increasingly contemporary researchers discuss it as a test about the fact that the surface of water remains horizontal despite the tilt of its container, thus the meaning of this test has drifted since it was originally devised by Piaget. It seems that girls demonstrate this principle at a later age than boys. In fact, it has been estimated that 40% of college women don't know the principle that the water level remains horizontal. This is a surprising result that has been replicated many times (Wittig & Allen, 1984). Robert and Chaperon (1989), for example, reported that 32% of college women and 15% of college men failed the Water-Level test.
