  "Water fountains" (WFs) are evolved objects showing high-velocity, collimated
jets traced by water maser emission. Most of them are in the post-Asymptotic
Giant Branch and they may represent one of the first manifestations of
collimated mass loss in evolved stars. We present water maser, carbon monoxide,
and mid-infrared spectroscopic data (obtained with the Australia Telescope
Compact Array, Herschel Space Observatory, and the Very Large Telescope,
respectively) toward IRAS 15103--5754, a possible planetary nebula (PN) with WF
characteristics. Carbon monoxide observations show that IRAS 15103-5754 is an
evolved object, while the mid-IR spectrum displays unambiguous [NeII] emission,
indicating that photoionization has started and thus, its nature as a PN is
confirmed. Water maser spectra show several components spreading over a large
velocity range ~75 km/s and tracing a collimated jet. This indicates that the
object is a WF, the first WF known that has already entered the PN phase.
However, the spatial and kinematical distribution of the maser emission in this
object are significantly different from those in other WFs. Moreover, the
velocity distribution of the maser emission shows a "Hubble-like" flow (higher
velocities at larger distances from the central star), consistent with a
short-lived, explosive mass-loss event. This velocity pattern is not seen in
other WFs (presumably in earlier evolutionary stages). We therefore suggest
that we are witnessing a fundamental change of mass-loss processes in WFs, with
water masers being pumped by steady jets in post-AGB stars, but tracing
explosive/ballistic events as the object enters the PN phase.
