The conceptualization of the horizontal was tested by having 128 university students draw the water surface on pictures of tilted glasses, as if the glasses were half-filled with water. Most Ss made errors of more than 5°, which suggests that adults have difficulty, much as children do, with the concept of horizontality as tested by this task.
SmedslundJ.The effect of observation on children's representation of the spatial orientation of a water surface. J. genet. Psychol., 1963, 102, 195–202.
