  We derive an explicit relationship between the moment of inertia and the
quadrupole response function of an interacting gas confined in a harmonic trap.
The relationship holds for both Bose and Fermi systems and is well suited to
reveal the effects of irrotationality of the superfluid motion. Recent
experimental results on the scissors mode are used to extract the value of the
moment of inertia of a trapped Bose gas and to point out the deviations from
the rigid value due to superfluidity.
