  Exact diagonalizations with a realistic interaction show that configurations
with four neutrons in a major shell and four protons in another -or the same-
major shell, behave systematically as backbending rotors. The dominance of the
$q\cdot q$ component of the interaction is explained by an approximate form of
SU3 symmetry. It is suggested that these configurations are associated with the
onset of rotational motion in medium and heavy nuclei.
