Folk Tales from around the world
A long time ago in China, a man of the state of Chu had a spear and a shield for sale in the market. He was loudly trying to sell his shield. "Hey, look, my shield is so strong that nothing in the world can pierce through it." He then shouted praises of his spear. "My spear is the strongest one; it can pierce through anything." "What would happen." he was asked by someone, "if you spear is used to pierce your shield?" The man in the end was stuck in an embarrassing situation.
It is impossible for the two best opposing weapons to coexist with no contradiction. The fable tells us not to say something self-contradictory.