  It is now well established that glasses feature quasilocalized nonphononic
excitations --- coined "soft spots"---, which follow a universal $\omega^4$
density of states in the limit of low frequencies $\omega$. All glass-specific
properties, such as the dependence on the preparation protocol or composition,
are encapsulated in the non-universal prefactor of the universal $\omega^4$
law. The prefactor, however, is a composite quantity that incorporates
information both about the number of quasilocalized nonphononic excitations and
their characteristic stiffness, in an apparently inseparable manner. We show
that by pinching a glass, i.e. by probing its response to force dipoles, one
can disentangle and independently extract these two fundamental pieces of
physical information. This analysis reveals that the number of quasilocalized
nonphononic excitations follows a Boltzmann-like law in terms of the parent
temperature from which the glass is quenched. The latter, sometimes termed the
fictive (or effective) temperature, plays important roles in non-equilibrium
thermodynamic approaches to the relaxation, flow and deformation of glasses.
The analysis also shows that the characteristic stiffness of quasilocalized
nonphononic excitations can be related to their characteristic size, a long
sought-for length scale. These results show that important physical
information, which is relevant for various key questions in glass physics, can
be obtained through pinching a glass.
