無根水 Water Without A Root
多官聞言道：「此物乃世間所無者。請問那一般引子是何？」行者道：「用無根水送下。」眾官笑道：「這個易取。」行者道：「怎見得易取？」多官道：「我這裡人家俗論：若用無根水，將一個碗盞，到井邊或河下，舀了水，急轉步，更不落地，亦不回頭，到家與病人吃藥，便是。」多官闻言道：「此物乃世间所无者。请问那一般引子是何？」行者道：「用无根水送下。」众官笑道：「这个易取。」行者道：「怎见得易取？」多官道：「我这里人家俗论：若用无根水，将一个碗盏，到井边或河下，舀了水，急转步，更不落地，亦不回头，到家与病人吃药，便是。」
逐句解 Line by line
Hearing this, the officials said, "These things are not to be had in this world.
They take the list seriously enough to answer it seriously, which is the joke. Their objection is that the ingredients cannot be obtained, not that the list is absurd. 間 is jiān.
"May we ask what the other vehicle is?"
那 is nà here, "that" — the question is being carried by 是何, so 那一般 means that other one of the two he mentioned, not "which one".
Pilgrim said, "Send it down with rootless water."
無根水 — water with no root — is a genuine term in Chinese medicine for rainwater caught before it lands. He gives it flatly, as though it were the obvious alternative to a crow's fart.
The officials laughed and said, "That one is easy to get."
Their relief is audible, and it is misplaced. They have understood the phrase to mean something quite different from what he means by it.
Pilgrim said, "How do you make it out to be easy to get?"
A trap, and he already knows the answer they will give. 見得 keeps the full dé.
The officials said, "By the common lore of the households hereabouts: for rootless water you take a bowl to a well or down to the river, dip up the water, turn on your heel at once, never let it touch the ground, never look back, and give it to the sick man at home with his medicine. That is it."
The folk procedure is described with real precision — the turning on the heel, the not looking back, the not letting it touch the ground. This is genuine popular practice, reported straight, and it is about to be disqualified on a technicality. 更 is gèng, and 落 already reads luò here, where two sites further on it will have to be argued for.
The officials object that the six things are not to be had in this world, which is true and is also the wrong objection. They have taken the list as a supply problem rather than as a joke, and politely ask for the other option.
Rootless water is a genuine term in Chinese medicine, and Pilgrim produces it flatly. The officials laugh with relief and call it easy — and then he asks them how they make it out to be easy, a question he asks because he already knows what they think it means.
Their answer is the longest speech in the passage and the most closely observed thing in it: take a bowl to a well or a river, dip it, turn on your heel at once, never let it touch the ground, never look back, carry it home to the sick man. Real popular practice, reported straight — and about to be ruled out on a technicality.