  The longstanding problem of modeling double-mode behaviour of classical
pulsators was solved with the incorporation of turbulent convection into
pulsation hydrocodes. However, the reasons for the computed double-mode
behaviour were never clearly identified. In our recent papers (Smolec &
Moskalik 2008a,b) we showed that the double-mode behaviour results from the
neglect of negative buoyancy effects in some of the hydrocodes. If these
effects are taken into account, no stable non-resonant double-mode behaviour
can be found. In these proceedings we focus our attention on the role of
negative buoyancy effects in classical Cepheid models.
