  Within the scenario of the potential energy landscape (PEL), a thermodynamic
model has been developed to uncover the physics behind the Angell plot. In our
model, by separating the barrier distribution in PELs into a Gaussian-like and
a power-law form, we obtain a general relationship between the relaxation time
and the temperature. The wide range of the experimental data in the Angell
plot, as well as the molecular-dynamics data, can be excellently fitted by two
characteristic parameters, the effective barrier ({\omega}) and the effective
width ({\sigma}) of a Gaussian-like distribution. More importantly, the fitted
{\omega} and {\sigma}^2 for all glasses are found to have a simple linear
relationship within a very narrow band, and fragile and strong glasses are well
separated in the {\omega}-{\sigma}^2 plot, which indicates that glassy states
only appear in a specific region of the PEL.
