Thieves always had a guilty conscience. It was a truth that lasted for an eternity.
Before Zhang Tie became a knight, even before he became a real fighter, he had heard a story from his mother which was popular among Hua people for long. It was about how an ancient Hua official painted ink on the deity's image so as to catch a thief. Despite being simple, that story had been widely spread among Hua people.
In a dark room, an official painted the deity's image with ink. After that, he told a lot of suspects that the deity image was pretty effective. As long as the suspects entered the room one after another and put their hands on the deity's image, that deity's image would tell him which one was the real thief.