  Geometrical frustration is a central challenge in contemporary condensed
matter physics, a crucible favourable to the emergence of novel physics. The
pyrochlore magnets, with rare earth magnetic moments localized at the vertices
of corner-sharing tetrahedra, play a prominent role in this field, with a rich
variety of exotic ground states ranging from the "spin ices" \hoti\ and \dyti\
to the "spin liquid" and "ordered spin ice" ground states in \tbti\ and \tbsn.
Inelastic neutron scattering provides valuable information for understanding
the nature of these ground states, shedding light on the crystal electric field
(CEF) level scheme and on the interactions between magnetic moments. We have
performed such measurements with unprecedented neutron flux and energy
resolution, in the "ordered spin ice" \tbsn. We argue that a new interaction,
which involves the spin lattice coupling through a low temperature distortion
of the trigonal crystal field, is necessary to account for the data.
