  Equations of gravity when projected on spacetime horizons resemble
Navier-Stokes equation of a fluid with a specific equation of state [1-3]. We
show that this equation of state describes massless ideal relativistic gas. We
use these results, and build an explicit and simple molecular model of the
fluid living on the Schwarzschild and Reissner-Nordstr\"om black hole horizons.
For the spin zero Bose gas, our model makes two predictions: (i) The horizon
area/entropy is quantized as given by Bekenstein's quantization rule, (ii) The
model explains the correct type of proportionality between horizon area and
entropy. However, for the physically relevant range of parameters, the
proportionality constant is never equal to 1/4.
