  We establish certain Gaussian type upper bound for the heat kernel of the
conjugate heat equation associated with 3 dimensional ancient $\kappa$
solutions to the Ricci flow.
  As an application, using the $W$ entropy associated with the heat kernel, we
give a different and shorter proof of Perelman's classification of backward
limits of these ancient solutions. The current paper together with \cite{Z:2}
and a different proof of universal noncollapsing due to Chen and Zhu
\cite{ChZ:1} lead to a simplified proof of the Poincar\'e conjecture without
using reduced distance and reduced volume.
