  When a lump of matter falls into a black hole it is expected that a
marginally trapped tube when hit moves outwards everywhere, even in regions not
yet in causal contact with the infalling matter. But to describe this
phenomenon analytically in 3+1 dimensions is difficult since gravitational
radiation is emitted. By considering a particle falling into a toy model of a
black hole in 2+1 dimensions an exact description of this non-local behaviour
of a marginally trapped tube is found.
