  "Why are the tin isotopes soft?" has remained, for the past decade, an open
problem in nuclear structure physics: models which reproduce the isoscalar
giant monopole resonance (ISGMR) in the "doubly-closed shell" nuclei, $^{90}$Zr
and $^{208}$Pb, overestimate the ISGMR energies of the open-shell tin and
cadmium nuclei, by as much as 1 MeV. In an effort to shed some light onto this
problem, we present results of detailed studies of the ISGMR in the molybdenum
nuclei, with the goal of elucidating where--and how--the softness manifests
itself between $^{90}$Zr and the cadmium and tin isotopes. The experiment was
conducted using the $^{94,96,98,100}$Mo($\alpha,\alpha^\prime$) reaction at
$E_\alpha = 386$ MeV. A comparison of the results with relativistic,
self-consistent Random-Phase Approximation calculations indicates that the
ISGMR response begins to show softness in the molybdenum isotopes beginning
with $A=92$.
