  We compare rigidity of materials in two phases, liquid and solid phases. As a
measure of the rigidity, we employ the one characterizing how firmly the
material is fixed by low density of pinning centers, such as impurities and
rough surfaces of walls, against a weak force. Although a solid is more rigid
than a liquid against a low-frequency force, we find that against a
high-frequency force the liquid becomes more rigid than the solid of the same
material. Since this result is derived from universal properties of a response
function, it is valid for wide classes of materials, including quantum and
classical systems and crystalline and amorphous solids. An instructive example
is studied using nonequilibrium molecular dynamics simulations. We find that
the frequency region in which a solid is more flexible than a liquid is not
purely determined by intrinsic properties of the solid. It depends also on
extrinsic factors such as the density of pinning centers.
