  We discuss various experiments on the time decay of velocity autocorrelation
functions in billiards. We perform new experiments and find results which are
compatible with an exponential mixing hypothesis, first put forward by [FM]:
they do not seem compatible with the stretched exponentials believed, in spite
of [FM], to describe the mixing. The analysis led us to several byproducts: we
obtain information about the normal diffusive nature of the motion and we
consider the probability distribution of the number of collisions in time $t_m$
(as $t_m\to\io$) finding a strong dependence on some geometric characteristics
of the locus of the billiards obstacles.
