  The anti-de Sitter/conformal field theory (AdS/CFT) correspondence implies
that small perturbations of a black hole correspond to small deviations from
thermodynamic equilibrium in a dual field theory. For gauge theories with an
Einstein gravity dual, the AdS/CFT correspondence predicts a universal value
for the ratio of the shear viscosity to the entropy density, $\eta/s=1/4\pi$.
It was conjectured recently that all fluids conform to the lower bound $\eta/s
\geq 1/4\pi$. This {\it conjectured} bound has been the focus of much recent
attention. However, despite the flurry of research in this field we still lack
a proof for the general validity of the bound. In this essay we show that this
mysterious bound is actually a direct outcome of the interplay between gravity,
quantum theory, and thermodynamics.
