個柳樹根 Nothing but a Willow Root
急掣金箍棒，撲的一下打破了。八戒看時，乃是個柳樹根。獃子忍不住罵起來道：「我把你這夥毛團！你將我師父藏在洞裡，拿個柳樹根哄你豬祖宗，莫成我師父是柳樹精變的。」急掣金箍棒，扑的一下打破了。八戒看时，乃是个柳树根。呆子忍不住骂起来道：「我把你这伙毛团！你将我师父藏在洞里，拿个柳树根哄你猪祖宗，莫成我师父是柳树精变的。」
逐句解 Line by line
He whipped out the gold-hooped rod and with one blow smashed it open.
撲的一下 is one sound-word and one stroke — the whole demonstration in four characters. The gold-hooped rod is not needed for this and he uses it anyway, which is characteristic.
Bajie looked, and it was a willow root.
Ten characters, and the deception collapses. The novel gives the discovery to Bajie rather than Wukong, which is generous — the one who was weeping gets to be the one who sees.
The Idiot could not hold himself in and started swearing: "You pack of hairy lumps!
毛團 is a ball of fur, and as abuse it means something like shaggy brutes — a fine insult from a man with a pig's head. Wukong will use the same word on the old fiend later in the chapter, so it is clearly a family favourite.
You have got my master hidden away in that cave, and you take a willow root to hoodwink your Grandfather Pig! I suppose next you will say my master was a willow spirit all along."
豬祖宗, Grandfather Pig, is Bajie doing the same self-promotion Wukong did at the gate, and it is funnier because he has less to back it with. 莫成 introduces a sarcastic suggestion — you are not going to tell me that, are you. The tree-spirit gibe is well chosen: this novel is full of vegetation that turns out to be people, and Bajie knows it.
The deception fails on the simplest possible test, and the fiends never considered it. They thought carefully about the tray, the blood, the apology and the form of address, and not at all about what the object would sound like when dropped.
Bajie promoting himself to Grandfather Pig immediately after Wukong announced himself as Grandpa Sun is the novel keeping its running joke in circulation. Both of them insist on being somebody's revered elder, and neither of them is behaving like one.
The sarcasm about willow spirits has teeth. In a book where trees regularly stand up and recite poetry, saying that somebody has swapped your master for a tree root is not as absurd as it sounds, and Bajie is pointing out that the fiends have not even bothered to make the lie plausible in a world where it easily could be.