  The recently proposed strategy for studying the equilibrium thermodynamics of
the glass phase using a molecular liquid is reviewed and tested in details on
the solvable case of the $p$-spin model. We derive the general phase diagram,
and confirm the validity of this procedure. We point out the efficacy of a
system of two weakly coupled copies in order to identify the glass transition,
and the necessity to study a system with $m<1$ copies ('clones') of the
original problem in order to derive the thermodynamic properties of the glass
phase.
