  The concept of "fragility", which was introduced to characterize the degree
of super-Arrhenius temperature dependence of the relaxation time and transport
coefficients, has since been taken as a key quantity that seems to correlate
with other properties of glass-forming liquids and polymers. With the goal of
assessing the usefulness and the robustness of the concept, we address here
several questions: How to best quantify fragility? How significant are the
observed differences in fragility? Is fragility connected to "cooperativity"
and collective behavior?
