A man from the state of Chu was selling a spear and a shield in the marketplace. To attract buyers, he first held up his shield and boasted loudly, "My shield is so strong that nothing in the world can pierce it!"
Then, he raised his spear and bragged, "And my spear is so sharp that it can pierce through anything!"
A bystander, hearing this, asked him, "If what you say is true, what would happen if you used your spear to strike your own shield?"
The seller was left speechless, unable to answer. He could only gather his weapons and leave.
This story gave rise to the Chinese idiom "zì xiāng máo dùn" (自相矛盾), which means "to contradict oneself" or "to be self-contradictory."