His Spear Against His Shield - Chinese Idiom Stories - Beijing Language and Cultural Center for Diplomatic Missions

A man of the state of Chu had a spear and a shield for sale. He was praising 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? “
He was unable to give an answer.
  It is impossible for the strongest shield to coexist with a spear that finds nothing impenetrable.