  This perspective article reviews arguments that glass-forming liquids are
different from those of standard liquid-state theory, which typically have a
viscosity in the mPa$\cdot$s range and relaxation times of order picoseconds.
These numbers grow dramatically and become $10^{12}-10^{15}$ times larger for
liquids cooled toward the glass transition. This translates into a qualitative
difference, and below the ``solidity length'' which is of order one micron at
the glass transition, a glass-forming liquid behaves much like a solid. Recent
numerical evidence for the solidity of ultraviscous liquids is reviewed, and
experimental consequences are discussed in relation to dynamic heterogeneity,
frequency-dependent linear-response functions, and the temperature dependence
of the average relaxation time.
