impenetrable shield

A man of the state of Chu had a spear and a shield for sale. He was loud in praises of his shield.
"My shield is so strong that nothing can pierce it through." 　　He also sang praises of his spear. 　　"My spear is so strong that it can pierce through anything." 　　"What would happen," he was asked, "if your spear is used to pierce your shield?" 　　It is impossible for an impenetrable shield to coexist with a spear that finds nothing impenetrable.