  A fundamental problem of glass transition is to explain the jump of heat
capacity at the glass transition temperature $T_g$ without asserting the
existence of a distinct solid glass phase. This problem is also common to other
disordered systems, including spin glasses. We propose that if $T_g$ is defined
as the temperature at which the liquid stops relaxing at the experimental time
scale, the jump of heat capacity at $T_g$ follows as a necessary consequence
due to the change of system's elastic, vibrational and thermal properties. In
this picture, we discuss time-dependent effects of glass transition, and
identify three distinct regimes of relaxation. Our approach explains widely
observed logarithmic increase of $T_g$ with the quench rate and the correlation
of heat capacity jump with liquid fragility.
