Once upon a time, a merchant in the state of Chu tried to sell a spear and a shield.

When asked, "How good is this shield?"

The merchant boasted, "No weapon can penetrate this shield, no matter how sharp the weapon is!"

When asked, "How good is this spear?"

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The merchant boasted again, "This spear can penetrate any shield, no matter how hard it is!"

When asked, "If you strike your shield with your spear, what would happen?"

The merchant could not answer.

The word 'spear' in Chinese is máo (矛) and the word 'shield' in Chinese is dùn (盾). Together, it makes the Chinese word for 'contradiction' máodùn (矛盾).

An idiom for this is zì xiāng máo dùn (自相矛盾) meaning 'use one's spear against one's own shield'.