  We present an insightful ``derivation'' of the Langevin equation and the
fluctuation dissipation theorem in the specific context of a heavier particle
moving through an ideal gas of much lighter particles. The Newton's Law of
motion ($m{\ddot x}=F$) for the heavy particle reduces to a Langevin equation
(valid on a coarser time scale) with the assumption that the lighter gas
particles follow a Boltzmann velocity distribution. Starting from the
kinematics of the random collisions we show that (1) the average force $<F>
\propto -{\dot x}$ and (2) the correlation function of the fluctuating force
$\eta= F-< F>$ is related to the strength of the average force.
