A man from the state of Chu was selling a spear and a shield. He praised his shield, saying, "My shield is so strong that nothing can pierce it."
He then praised his spear, boasting, "My spear is so sharp that it can pierce through anything."
Someone then asked him, "What would happen if you used your spear to strike your shield?" The man was unable to answer.
It is impossible for an impenetrable shield to coexist with a spear that can pierce anything.