  A martingale transform $ T$, applied to an integrable locally supported
function $ f$, is pointwise dominated by a positive sparse operator applied to
$ \lvert f\rvert $, the choice of sparse operator being a function of $ T$ and
$ f$. As a corollary, one derives the sharp $ A_p$ bounds for martingale
transforms, recently proved by Thiele-Treil-Volberg, as well as a number of new
sharp weighted inequalities for martingale transforms. The (very easy) method
of proof (a) only depends upon the weak-$L^1$ norm of maximal truncations of
martingale transforms, (b) applies in the vector valued setting, and (c) has an
extension to the continuous case, giving a new elementary proof of the $ A_2$
bounds in that setting.
