有個蟹精 There Was a Crab Spirit
老龍道：「家樂兒斷乎不敢，可曾有甚生人進來？」龍子、龍孫道：「適才安座之時，有個蟹精到此，那個便是生人。」牛王聞說，頓然省悟道：「不消講了。老龙道：「家乐儿断乎不敢，可曾有甚生人进来？」龙子、龙孙道：「适才安座之时，有个蟹精到此，那个便是生人。」牛王闻说，顿然省悟道：「不消讲了。
逐句解 Line by line
The old dragon said, "The household musicians would certainly not dare. Was there perhaps some stranger who came in?"
家樂 is a private troupe of musicians kept in a great household, and its 樂 is yuè; 兒 in 家樂兒 is the swallowed suffix er. 曾 is céng in 可曾 — the "have you ever" reading — and not the family name it looks like. 甚 is shén, the question word.
The dragon sons and dragon grandsons said, "Just now, at the time of seating, there was a crab spirit who came here. That one was a stranger."
子 in 龍子 is the full zǐ. They answer without knowing what they are answering, exactly as the Rākṣasī did.
The Bull King, hearing it, understood on the instant and said, "There is no need to say more."
省悟 takes xǐng, the waking-up reading. 頓然 makes the recognition instantaneous: he needs one word.
Three words undo the whole imposture. A crab spirit came in at the seating; that one was a stranger; and the Bull Demon King understands at once. He fought the Great Sage that morning, knows his reputation for changing shape, and needs to hear only that an unaccountable creature was present.
The novel enjoys the symmetry of the deduction. The crab was arrested for walking sideways among the guests, tried, sentenced on paper and sent outside to wait — and every one of those facts is now evidence. The court's own procedure has produced a perfect record of the thief's movements.