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.
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