剝皮亭內 Inside the Flaying Pavilion
我們那剝皮亭內有吃不了的人頭選一個來。」眾妖即至亭內揀了個新鮮的頭，教啃淨頭皮，滑塔塔的，還使盤兒拿出，叫：「大聖爺爺，先前委是個假頭。我们那剥皮亭内有吃不了的人头选一个来。」众妖即至亭内拣了个新鲜的头，教啃净头皮，滑塔塔的，还使盘儿拿出，叫：「大圣爷爷，先前委是个假头。
逐句解 Line by line
Go and pick one out from the heads in our flaying pavilion that we have not got through eating."
剝皮亭 is a flaying pavilion, and the novel drops it in without a flicker of emphasis — the cave has a dedicated outbuilding for the purpose and a surplus stock. In 吃不了, more than can be finished, 了 is liǎo, to get to the end of something, rather than the small grammatical marker the same character carries everywhere else.
The fiends went straight to the pavilion, picked out a fresh head, had the skin gnawed clean off it until it was slippery and smooth, and carried it out on the tray again, calling, "Grandpa Great Sage, the earlier one was indeed a false head.
滑塔塔 is slippery with the second character doubled, one of those sound-and-texture words the novel scatters through its descriptions. The skinning is not squeamishness on the fiends part but forgery technique: a recognisable face would give the game away, and a skull will not. They have learned from being caught, and what they learned is to destroy more evidence.
The single most chilling detail in the chapter is a word about storage. The fiends are not hunting for a head; they are choosing one, from a supply, in a building kept for the purpose. Everything the reader needs to know about this cave is in the phrase about heads they have not got through eating.
Their second attempt is better than their first in every technical respect, and it is better because Wukong taught them. He proved the first head was wood; they have now brought bone. He would have recognised a stranger's face; they have removed it. Their own failure taught the forgers their craft.