  A new mechanism for the formation of cometary tails behind dense clouds or
globules is discussed. Numerical hydrodynamical models show that when a dense
shell of swept-up matter overruns a cloud, material in the shell is focussed
behind the cloud to form a tail. This mode of tail formation is completely
distinct from other methods, which involve either the removal of material from
the cloud, or shadowing from a strong, nearby source of ionization. This
mechanism is relevant to the cometary tails seen in planetary nebulae and to
the interaction of superbubble shells with dense clouds.
