For every bounded
planar domain $D$ with a smooth boundary, we define a ``Lyapunov
exponent'' $\Lambda(D)$ using a fairly explicit formula. We
consider two reflected Brownian motions in $D$, driven by the same
Brownian motion (i.e., a ``synchronous coupling''). If
$\Lambda(D)>0$ then the distance between the two Brownian
particles goes to $0$ exponentially fast with rate $\Lambda
(D)/(2|D|)$ as time goes to infinity. The exponent $\Lambda(D)$ is
strictly positive if the domain has at most one hole. It is an
open problem whether there exists a domain with $\Lambda(D)<0$.