The earliest Chinese kites were made of wood and called muyuan (wooden kites); they date as far back as the Warring States Period (475-221 B.C.) at least two millennia ago. After the invention of paper, kites began to be made of this new material called zhiyuan (paper kites).Instead of being playthings, early kites were used for military purposes. Historical records say they were large in size; some were powerful enough to carry men up in the air to observe enemy movements, and others were used to scatter propaganda leaflets over hostile forces. According to the Records of Strange Events (Du Yi Zhi), an ancient work, when Xiao Yan, Emperor Wudi (464-549) of the Liang Dynasty, was surrounded at Taicheng, Nanjing by the rebel troops under Hou Jing, it was by means of a kite that he sent out an S.O.S. message for outside help.