A drop of water thrown on a super-hydrophobic solid will often bounce off. Here we discuss
the conditions to be fulfilled on the surface design (which provides super-hydrophobicity) to
observe such a behavior. This allows us to precise how a material can be made water-
repellent. We show in particular how the reduction of the scale of the microstructure provides
a robust water repellency, and describe some peculiarities of violent shocks on such
surfaces.