Ring galaxies are dramatic examples of galaxy transformation caused by a
remarkably simple interaction. Observations and numerical models
(Lynds & Toomre 1976)
argue persuasively that these objects are formed by the near central
passage of a companion through a spiral along the rotation axis. The
brief additional gravitational force
induces epicyclic motions throughout the disk, which act to form radially
propagating orbit-crowded rings of gas and stars. The concentration of
the ISM into the expanding ring (at the disk's expense) is nearly total
and can last for 
400
Myrs. It is this radical rearrangement of the spiral's ISM that is
responsible for their interesting star forming properties.