It is well-known that supercooled liquid shows behavior different from that of simple liquid. We show this behavior actually has nothing to do with supercooling, but is an intrinsic property of any liquid associated with dynamic cooperativity crossover related to the lifetime of phonon. Above the cooperativity crossover temperature, TA, atoms are dynamically uncorrelated. But below TA atomic dynamics become cooperative and correlated in space and time. We propose to call the liquid below TA “cooperative liquid”. Viscosity increases rapidly with increased cooperativity, culminating in the glass transition. Therefore TA is a more fundamental temperature than the glass transition temperature Tg. Externally applied stress suppresses cooperativity, leading to decrease in viscosity and glass-to-liquid transition, resulting in mechanical failure. We discuss simulations and scattering experiments which support this view. This work is supported by the Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.