How to hint that something has a weakness to water?

[SangoProduction]

So, for a boss monster of a cave (gargantuan animated object, basically), I've basically buffed up defenses considerably, unless they soak it with water.

How do I make that obvious that water can be used to soften its defenses, without having it literally written on the walls "Water = win"? Puzzles aren't really puzzles if they don't have clues. (They said they like puzzles but prefer combat. So why not both?)

[Hackulator]

So, for a boss monster of a cave (gargantuan animated object, basically), I've basically buffed up defenses considerably, unless they soak it with water.

How do I make that obvious that water can be used to soften its defenses, without having it literally written on the walls "Water = win"? Puzzles aren't really puzzles if they don't have clues. (They said they like puzzles but prefer combat. So why not both?)

Put puddles of water in the cave, have the monster go out of it's way to avoid the puddles, even though they're not deep enough to actually slow down a creature.

Also if its animated I assume its made of something that clearly would not do well when getting wet?

[Gildedragon]

Dust. Have the place be very very dry. Some dessication damage early on. Bits of cloth and flesh and bone are preserved, mummified by the dryness

[SangoProduction]

Put puddles of water in the cave, have the monster go out of it's way to avoid the puddles, even though they're not deep enough to actually slow down a creature.

Also if its animated I assume its made of something that clearly would not do well when getting wet?

Yeah. Dirt / Sand. I mean, the composition alone probably is not enough to suggest use of water, so I'm coming to the playground to get ideas for how to make it more obvious...kinda last minute, as I started thinking about that.

[Mendicant]

Is this monster totally unique in-universe? Is it conscious of its vulnerability? What is it made of?

Some ideas off the top of my head:
Other, smaller monsters of a similar kind have the same vulnerability.
Other, smaller monsters of a similar kind always have *some* sort of vulnerabilit
A Skyrim-style journal that contains an unsolved riddle, or a premise ("Old Fuzzwinkle the Enchanter *always* gave his creations a weakness! I just have to deduce what.")
A body of water that the thing studiously avoids. Maybe that body of water leads to other parts of the cave the boss monster would obviously like to go to, but doesn't.
Obvious signs that the creature is slightly corroded. EDIT: If it's made of sand, perhaps pockmarks on its shoulders and the top of its head from dripping water.

[SangoProduction]

Is this monster totally unique in-universe? Is it conscious of its vulnerability? What is it made of?

Some ideas off the top of my head:
Other, smaller monsters of a similar kind have the same vulnerability.
Other, smaller monsters of a similar kind always have *some* sort of vulnerabilit
A Skyrim-style journal that contains an unsolved riddle, or a premise ("Old Fuzzwinkle the Enchanter *always* gave his creations a weakness! I just have to deduce what.")
A body of water that the thing studiously avoids. Maybe that body of water leads to other parts of the cave the boss monster would obviously like to go to, but doesn't.
Obvious signs that the creature is slightly corroded. EDIT: If it's made of sand, perhaps pockmarks on its shoulders and the top of its head from dripping water.

Very much appreciated. I was thinking something like a few of those.

[Telok]

Salt. Put the critter in a salt mine and play up how thirsty the dry air is making the characters.

[JustIgnoreMe]

Any/all of the above, plus: a riddle.

I stole this from Dragon Magazine, I think:

"Horses and kings share these perforce, But [monster] wants none of this, of course."

Answer is reins/reigns/rain.