Self-contradictory

Self-contradictory

Long long ago there is a person sale spear and shield on the street in the Chu state.For contrat the buyers,he hold the shield said that:"My shield is so hard that there are no weapons can pierce it!"
Then he put down the shield and hold the spear said boastly that:"My spear is so sharp that it can pierce eveything!"At the moment,somebody asked him："What is the result if take your spear pierce your shield?"The seller can not say a word.
Meaning:If exaggerated something's stong points divorced from reality should be creat self-contradictory joke constantly.