ABSTRACT
All children like sliding things down slopes-or having a slide themselves-and even our four-year-old subjects were proficient at this. However, do they know how to make some thing slide down a large inclined sheet of cardboard along a predetermined path? In other words, can they correctly orient the cardboard so as to achieve this, or arrange bricks so that a sheet of cardboard placed on them will be tilted the right way? The aim of this research was to determine how and why children's physical skill in this area evolves, and then to study their cognizance of the problem. Thus we hoped to find out how the children progress to the stage of saying, like one of our twelve-year-olds: It always goes down to the lowest side (= finds the line of greatest slope), revealing a grasp of the principle of extremum.
