To suppress opposition, Empress Wu Zetian of the Tang Dynasty appointed ruthless judges, among whom Zhou Xing and Lai Junchen were notoriously brutal. They framed and tortured many upright officials and commoners.
Once, Wu Zetian received a letter accusing Zhou Xing of plotting rebellion. Furious, she ordered Lai Junchen to investigate severely. Lai Junchen was apprehensive; he knew Zhou Xing was too cunning to confess based on a mere letter, and failure would bring the Empress's wrath. He devised a plan.
Lai Junchen invited Zhou Xing to a sumptuous feast. After several rounds of wine, Lai sighed, "I often encounter prisoners who stubbornly refuse to confess. Do you have any effective methods?" Zhou Xing, smiling insidiously, took a sip and said, "Get a large vat, scorch it with charcoal fire all around, and then ask the prisoner to step in. Will he still refuse to confess?"
Lai Junchen nodded in approval. He immediately had a vat brought in and a charcoal fire lit around it as described. Then he turned to Zhou Xing and said, "Someone at court has informed against you for plotting rebellion. The Empress has ordered me to investigate severely. So I beg your pardon, but would you kindly step into the vat?"
Upon hearing this, Zhou Xing dropped his wine cup in shock. He knelt down, nodded repeatedly, and confessed, "I am guilty. I confess I am guilty."
This story from Sima Guang's Comprehensive Mirror for Aid in Government gave rise to the idiom "kindly step into the vat," meaning to make someone suffer the same scheme they devised for others.