  We demonstrate that the changes in the elastic properties of the FeAs
systems, as seen in our resonant ultrasound spectroscopy data, can be naturally
understood in terms of fluctuations of emerging nematic degrees of freedom.
Both the softening of the lattice in the normal, tetragonal phase as well as
its hardening in the superconducting phase are consistently described by our
model. Our results confirm the view that structural order is induced by
magnetic fluctuations.
