原文：
李惠者，字德明，赵郡人也。少以才学知名，仕齐为中书舍人。齐亡入周，为太常博士。隋开皇中，累迁至吏部侍郎。性清廉，所居无长物。尝有门生献钱千文，惠叱之曰：“吾与卿有师生之义，岂可以此相污？”其廉洁如此。
后为扬州总管，时有商人郑克，因事被诬陷，狱吏欲以酷刑逼供。惠闻之，召郑克至，问曰：“汝何不自辩？”克对曰：“小人无罪，但恐官府不信。”惠笑曰：“若果无罪，何须惧之？今吾与汝各执一端皮，汝执黑皮，吾执白皮，若皮真，则汝无罪；若皮伪，则汝有罪。”
于是命人将两块皮置于水中，片刻后取出，黑皮湿而白皮干。惠谓克曰：“黑皮湿，是水之所渍；白皮干，是火之所炙。今吾白皮干，汝黑皮湿，可见汝皮之真伪矣。”遂释放郑克，并严惩诬告之人。
问题：
1. 李惠为何被称为“德明”？
2. 郑克是如何被证明无罪的？
答案：
1. 李惠被称为“德明”，是因为他品德高尚，清廉正直。
2. 郑克被证明无罪是因为李惠通过实验验证了皮的真假，从而确认了郑克的无辜。
翻译：
Li Hui, whose courtesy name was Deming and came from Zhao County, was known for his talent and learning since he was young. He served as a Zhongshu Shiren in the Qi Dynasty and later became a Tai Chang Doctor when Qi fell and he joined the Zhou Dynasty. During the reign of Emperor Wen of Sui, he was promoted to the position of Ministry of Personnel Deputy Minister. Li Hui was known for his integrity; there were no unnecessary possessions in his home. Once, one of his students offered him a thousand coins, but he scolded them saying: "I have a teacher-student relationship with you. How can this corrupt us?"
Later, he became the governor of Yangzhou. There was a merchant named Zheng Ke who was falsely accused and the prison officials wanted to use cruel torture to force a confession. Upon hearing this, Li Hui summoned Zheng Ke and asked: "Why don't you defend yourself?" Zheng replied: "I am innocent, but I fear the authorities will not believe me." Li Hui smiled and said: "If you are indeed innocent, why be afraid? Now, let each of us hold a piece of leather, you take the black one, I take the white one. If the leather is genuine, then you are innocent; if it is fake, then you are guilty."
He then ordered someone to place the two pieces of leather in water. After a while, they were taken out. The black leather was wet while the white leather remained dry. Li Hui told Zheng Ke: "The black leather is wet because it has been soaked in water; the white leather remains dry because it has been burned by fire. Since my white leather is dry and your black leather is wet, it is clear that the leather is genuine or fake." Thus, Zheng Ke was released and the accusers were severely punished.
This story highlights Li Hui's wisdom and integrity in resolving cases fairly and justly.
