In ancient times, there was a man who sold spears as well as shields.
One day, he hawked his wares at the market and was very pleased with himself. He held up a shield and said: "My shield is very strong. No matter how sharp a spear is, it cannot pierce my shield."
After that, he held up his spear and bragged: "My spear is very sharp. No matter how strong a shield is, my spear can pierce it.
When the people standing by heard this, they laughed up their sleeves. One of them asked him:
"According to what you said, your spear is the sharpest. No matter how strong a shield is, your spear can pierce it. Your shield is also the strongest. No matter how sharp a spear is, it cannot pierce your shield. Now, if I poke your shield with your spear, what will be the result?"
The man was so embarrassed that he could not utter a single word.