自相矛盾

Meaning: Self-contradictory (idiom)

During the Warring States Period, there was a man who sold shields and spears. He boasted that the shields he sold were so strong that nothing could pierce it. And he boasted of the sharpness of the spears which he had sold, and that nothing that could not be pierced by them. Someone on the road heard it and couldn't help saying, "What would happen if your spear was used to pierce your shield?" The man from Chu country stood aghast and could not answer his question.

战国时楚国有一个卖盾和矛的人，他夸说自己所卖的盾坚固无比，没有东西能把它刺穿；又夸说自己所卖的矛十分锋利，没有东西不被它刺穿。路上有人听见后，忍不住说：“如果用你的矛去刺你的盾，结果会如何？”楚国人立刻瞠目结舌，无法回答他的问题。