Response functions - like the dielectric constant - express how a medium reacts to an external perturbation. The medium we are interested in is finite temperature matter of quarks and gluons and the perturbation is shear in flow. For a very special conformally invariant (completely scalefree) matter the problem has a remarkable solution: the response functions can be computed from 5-dimensional gravity. For infinite wave length and zero frequency this leads to the celebrated prediction of viscosity/entropy=hbar/(4pi). I outline the method and describe how it can be implemented for all wave lengths and frequencies and also how extensions to the physically relevant case of QCD matter with scales, hadron masses and critical temperatures, could be modeled.