  Contradicting Oneself 自相矛盾

  In ancient times, there was a man who sold spears and shields. He used to boast, "My spears are the sharpest things in the world. They can penetrate anything." A moment later he would boast, "My shields are the toughest things in the world. Nothing can penetrate them." One day, a passerby asked him: "What would happen if you threw one of your spears at one of your shields?" 
  古代有个卖矛和盾的人。他一手举起他的矛夸耀说："我的矛锋利无比，无论多么坚硬的东西都能刺穿。"过了一会儿，他又举起另一只手里的盾说："我的盾坚硬无比，无论多么锋利的东西也刺不破。"观众中有一个人问他："用你的矛刺你的盾，结果会怎样呢？"这个人被问得哑口无言了。

  This idiom, "contradicting oneself", and the noun, contradiction, all came from the above story.
  "自相矛盾"用来比喻说话或做事不一致，互相抵触。