  Given a homogeneous Poisson process on ${\mathbb{R}}^d$ with intensity
$\lambda$, we prove that it is possible to partition the points into two sets,
as a deterministic function of the process, and in an isometry-equivariant way,
so that each set of points forms a homogeneous Poisson process, with any given
pair of intensities summing to $\lambda$. In particular, this answers a
question of Ball [Electron. Commun. Probab. 10 (2005) 60--69], who proved that
in $d=1$, the Poisson points may be similarly partitioned (via a
translation-equivariant function) so that one set forms a Poisson process of
lower intensity, and asked whether the same is possible for all $d$. We do not
know whether it is possible similarly to add points (again chosen as a
deterministic function of a Poisson process) to obtain a Poisson process of
higher intensity, but we prove that this is not possible under an additional
finitariness condition.
