One day, a weapon merchant arrives in a new village and tries to sell his spear and shield.
One villager walks up to him and asks how good his spear, he says that his spear can pierce any shield. Then, when asks how good his shield, he says that it can defend from all spear attacks.
Then the villager asks him what will happen if he takes his spear to strike his shield. The merchant can not answer. So, the merchant picks up the spear and kill the villager.
The lesson behind this story:
If someone asks you a different question you cannot answer, just kill the one ask the question.