Long long ago, a man was selling a spear and a shield in the market. He wanted to sell his goods quickly, so he was loud in praise of his shield,'' My shield is the strongest shield in the world. My shield is so strong that nothing in the world can pierce it,'' He was also loud in praise of his spear.'' My spear is the sharpest spear in the world. It can pierce anything in the world.'' At this time, an old man in the market asked him,'' The spear is the best weapon in the world as you said, then what would happen if you used your spear to pierce your shield'' The man could not give a good answer, It is impossible for the sharpest spear to pierce the strongest shield in the world.