<p>During the zhan Guo, there was a man in Chu who sold spears and shields. He boasted first that his shield was very hard and said, "No matter what you use, you can’t destroy it!" Then he boasted that his spear was very sharp and said, "Anything can be destroyed it!" The man asked him, "if you use your spear to stab your shield, what will they do?" The man could not answer. The crowd laughed at him. </p> <p>This story told us there can be no unbreakable shield and unbreakable spear in the world. The Chu state unilaterally exaggerated the function of spear and shield, resulting in a situation that cannot be justified.</p>