International Tea Day is observed annually on May 21. It aims to raise awareness of the long history and the deep cultural and economic significance of tea worldwide. Do you know what Pu'er tea is? A characteristic of this tea is that it mellows over the years, a bit like wine, due to the post-fermentation process. Where does the tradition of compressing Pu'er tea originate from, and what does Chama Gudao mean? Join CGTN to visit Wuliang Mountain in SW China's Yunnan and explore the rich history of Pu'er tea.