  We report on the expansion of a Fermi-Fermi mixture of Li-6 and K-40 atoms
under conditions of strong interactions realized near the center of an
interspecies Feshbach resonance. We observe two different phenomena of
hydrodynamic behavior. The first one is the well-known inversion of the aspect
ratio. The second one is a collective expansion, where both species stick
together and despite of their different masses expand jointly. Our work
constitutes a first step to explore the intriguing many-body physics of this
novel system.
