The Swiss scientist Jean Piaget (1896-1980) spent his professional career studying the cognitive development of young children.  After years of study Piaget concluded that we all pass through stages of cognitive development.  This culminates with what is known as the Formal Operational Stage.  At this last stage the individual is able to use symbolic representation in abstract thought.  Piaget believed that the Formal Operational Stage should be developed in adolescence, although Piaget admitted not all adolescence reach this stage. 

It has been my experience that most students have not entered the Formal Operational Stage out of high school (try this test of Formal Operational thinking).  The question is how can we get students to operate at this highest level of thinking?  The answer is to get students to use abstract thought as much as possible.  Give students exercises that allow them to logically think through hypothetical situations (i.e., what would happen if gravity operated in reverse?)