However, this makes the U.S.
capable of embracing and supporting many models of teaching and schooling, with each still able to
reach critical
mass.
As water can exist in several distinct states (solid, liquid or gas) and move from one to another, a human society may also be seen as a material
capable of undergoing these changes of states as it
reaches critical
mass in terms of density of settlement, amount of energy consumed or even intensity of interaction.