  We prove monotonicity of a parabolic frequency on manifolds. This is a
parabolic analog of Almgren's frequency function. Remarkably we get
monotonicity on all manifolds and no curvature assumption is needed. When the
manifold is Euclidean space and the drift operator is the Ornstein-Uhlenbeck
operator this can been seen to imply Poon's frequency monotonicity for the
ordinary heat equation. Monotonicity of frequency is a parabolic analog of the
19th century Hadamard three circles theorem about log convexity of holomorphic
functions on $\CC$. From the monotonicity, we get parabolic unique continuation
and backward uniqueness.
