自相矛盾 — “To contradict oneself”

In the ancient State of Chu, there was a man selling weapons in the market: spears and shields. Many people gathered to watch.

He held up his shield and boasted to everyone, saying, "My shield is the strongest in the world; no matter how sharp or pointed a weapon is, it cannot pierce it!"

Then, the vendor picked up a spear and shamelessly bragged, "My spear is the sharpest in the world; no matter how firm or solid an object is, it cannot withstand a single thrust from it. As soon as it touches something, it will be pierced immediately!" He was very proud and then shouted loudly, "Come and see, come and buy! The strongest shield and the sharpest spear in the world!"

At this moment, a spectator stepped forward, picked up a spear, and then picked up a shield, asking, "What would happen if I used this spear to pierce this shield?"

"This—" The onlookers were stunned for a moment, then burst into a roar of laughter and dispersed. The weapons vendor, crestfallen, carried his spear and shield and left.