The perpetual resource concept is a complex one because the concept of resource is complex and changes with the advent of new technology (usually more efficient recovery), new needs, and to a lesser degree with new economics (e.g. changes in prices of the material, changes in energy costs, etc.). On the one hand, a material (and its resources) can enter a time of shortage and become a strategic and critical material (an immediate exhaustibility crisis), but on the other hand a material can go out of use, its resource can proceed to being perpetual if it was not before, and then the resource can become a paleoresource when the material goes almost completely out of use (e.g. resources of arrowhead-grade flint). Some of the complexities influencing resources of a material include the extent of recyclability, the availability of suitable substitutes for the material in its end-use products, plus some other less important factors.