In order to suppress those who were against her, Wu Zetian, the empress of the Tang Dynasty (618-907), appointed a few merciless persons to be judges. Two of them were extremely brutal; one was called Zhou Xing and the other was called Lai Junchen. They killed many upright civil and military officials as well as ordinary people by framing up cases against tham and by administering inhuman corporal punishment to them.
Once, Wu Zetian received a letter which informed against Zhou Xing. The letter aside that Zhou Xing was plotting a rebellion in collaboration with others. Wu Zetian was furiously angry when she read the letter, and immediately ordered Lai Junchen to deal with the case severely. Hearing the order, Lai Junchen had misgivings about it. He knew that Zhou Xing could not be forced to tell the truth merely by using a letter informing against him because he was very sly and crafty. Lai Junchen also knew that he would not be for given if he should fail in dealing with the case, because the empress would certainly blame him and punish him. How could he solve the problem then? He turned the problem over and over in his mind, and finally thought out a "brilliant scheme".
Lai Junchen had a sumptuous feast prepared, and invited Zhou Xing to his home. The two of them urged each other to drink, and they talked while drinking. After the wine had gone round three times, Lai Junchen pretended to sigh, "When Ihandle cases in ordinary times, I often com across prisoners who stubbournly refuse to admit they are guilty. I wonder if you have any effetive measures." Hearing this, he took a sip of the wine. Lai Junchen immediately pretended to be very earnest, saying, "Oh, please do tell me at once." Smiling insidiously, Zhou Xing said, "Get a big vat, scorch it hot with charcoal fire all around, and then let the prisoner come into the vat. Will the prisoner fail to make a confession of his crime?" Hearing this. LaiJunchen nodded his head in approval repeatedly. He then ordered his subordinates to bring a big vat, and had a charcoal fire lit all around it as Zhou Xing had said. He then turned to Zhou Xing and said, "Someone in the imperial court has informed against you, saying that you are plotting a rebellion. The empress has ordered me to deal with the case severely. So I beg your pardon, but would you kindly step into the "Someone in the imperial court has informed against you, saying that you are plotting a rebellion. The empress has ordered me to deal with the case severely. So I beg your pardon, but would you kindly step into the vat?" Hearing this, Zhou Xing dropped his wine cup to the ground and the cup broke with a crash. Then he knelt down with a flop, nodded repeatedly and said, "I am guilty. I confess I am guilty."
This story appears in A General History as a Mirror of Past Events by Sima Guang of the Song Dynasty. From this story, people have derived the set phrase "kindly step into the vat -- try what you have devised against others".