I'm looking for an old short story about an alien that turns out to be human. In the end the natives on the planet trap the human and spray him with the strongest solvent their scientists have ever discovered, water.
*"You threaten the Surviving One?" the mind-voice queried coldly. "Perhaps it will be as well if we proceed at once to pre-digestion. Very well, fellows, melt it down."
At once a fine spray of cool moisture enveloped the Terran. The fluid appeared to be expelled in minute droplets from pores covering the surface of leaves and stems alike. A drop trickled down Retief's upper lip, as the voice spoke again:
"You, O formerly Motile One, are now enveloped in a cloud of the most corrosive substance in nature. Prepare to be dissolved."
"That wouldn't be H2O, I suppose," Retief hazarded as his tongue touched the droplet on his lip.
"Precisely. Our methods of preparing nourishment are unparalleled. We ourselves are of course impervious to this caustic compound."
"I dare you to step up the volume," Retief said.
The swiftly evaporating mist had lowered the temperature to a bearable level, and his heat-parched skin was eagerly absorbing the water, which was now trickling down in an increased volume.
"You presume, O Motile One, to attempt to resist the corrosive action of the universe's most potent solvent?"
"Sure," Retief said. "I don't have time to be dissolved right now. If you boys are hungry, I'm in a position to offer you a full cargo of gourmet delights, if it isn't splashed all over the landscape, that is; or even if it is. I don't suppose you'd object to having to collect it."*
h/t to Geoff's answer for pointing me in the right direction.