The Juvenis Party has figured out that the well contains water that will enable them to walk through poisonous gas and continue onwards into the dungeon.
Did anyone guess? How long did it take. Were you following it and I just gave it away right now? I admit, the idea is pretty old school, not really my usual thing. I don't like puzzles, largely because people tend to overthink them and then we're going around in circles for ages.
I try to suspend overthinking by simply identifying it when it happens or jumping ahead to produce the experiment and demonstrate it has failed.
Whenever possible, I will try to emphasize the correct solution if a player mentions it in passing, as I did in this case. I had an NPC repeat a specific question stated by the player, and that broke the deadlock.
Am I wrong to do this? Say so if you feel I am. I do so because it is often necessary to make the game move forward when the players begin to feel lost.
If you haven't seen it yet, there's a comic.