zì xiāng máo dùn

Literal: To be both the spear (矛, máo) and shield (盾, dùn)

Actual: To contradict oneself

There once was a weapons dealer in ancient China who had goods to sell. Not unlike contemporary America, ancient China had a poor system of arms control. People had to gear up in case the Xiongnu invaded, or if the Emperor started losing the Mandate of Heaven by throwing lavish parties. The weapons dealer approached a quiet town and started promoting his wares, hollering as he rang his gong: “Gather round!”

“I possess a fine spear - a spear so strong, it can penetrate the strongest armor. A must for fighting off the Huns - and surprisingly handy for pole vaulting!” The townsfolk stood quietly, not quite believing the salesman. They weren’t really into gymnastics, apparently.

Sensing hesitation, the disheartened merchant continued with a new spiel: “Now look here instead: a strong, sturdy shield that protects against all incursions - invading Jurchen Tribes, debt collectors, uncomfortable political ideas that aren’t yours. You want it away? This shield will keep it at bay!”

At this point, one of the villagers asked: “What happens if I take your spear and ram it into your shield? Would the spear snap? Or would the shield shatter?”

Unlike in contemporary China, double-think was not part of the public consciousness just yet - and the villagers realized how silly the merchant was. And thus, amidst laughter and ridicule, the merchant shamefully fled the town.

TL;DR: Merchant fails to sell his unbreakable spear and impenetrable shield. Rumor has it that, with his talent for hypocrisy, the merchant became a successful congressman.