鬻矛誉盾 — “To contradict oneself”

During the Warring States period, Han Feizi once told a story about a man who sold spears and shields.

He would boast to people that his spear was the sharpest in the world, capable of piercing through anything.

Soon after, he would promote his shield, claiming it was the toughest in the world, impenetrable by anything.

Someone in the crowd then asked him, 'If you use your spear to pierce your shield, what would happen?'

The man had no answer and could only flee in embarrassment.