(Chinese idiom stories 1) 自相矛盾 zì xiāng máo dùn

Once upon a time there was a weapon seller in Chu. One day he went to the market to sell his spears and shields. He held up a shield and cried, “My shields are the strongest in the world. No spear, no matter how sharp it is, can penetrate them!” Then he held up a spear and shouted, “My spears are the sharpest in the world. They can break everything, no matter how hard it is!” Many people were drawn to see his spears and shields.
An onlooker picked up one of the spears and asked, “This spear can break everything?” asked one of the onlookers, picking up one of the spears. [1]
The weapon seller nodded.
The onlooker then picked up one of the shields and asked, “Nothing can break this shield?” The onlooker then picked up one of the shields and asked, [2]
The weapon seller nodded again.
“Then what will happen when I try to penetrate your shield with your spear?”
The weapon seller was struck dumb.


自相矛盾 zì xiāng máo dùn: to contradict oneself; to beat oneself with one's own staff

Note: Chu was one of the many nations during the Spring and Autumn (770-476 BC) and Warring States (475-221 BC) periods in Chinese history.

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[1] Why did I rearrange your sentence?  Because all prior quotes were by the seller. If you then start a new quote, the reader will presume that the seller is continuing to talk.  But that is not the case.  So, I have introduced the onlooker, and made it clear that it is now the onlooker talking.

[2] Why did I rearrange this sentence?  For two reasons.

First, you had just described the seller's reaction (of nodding his head).  If you then start a quote, the reader may think that the seller is now talking.  But that is not the case.

Second, you can finish a sentence with "he said" or "he asked" if these words appear immediately after the quote.  For example:

“I like this shield,” he said.

However, this format does not work if there is a narrative description between the quote and "he said" or "he asked".

“I like this shield,” he picked up the shield and said.  (This is wrong.)

Rather, you must do the following:

“I like this shield,” he said, picking up the shield.

Yet that format is still rather awkward.  It is cleaner to introduce the person, his action, and his quote:

He picked up the shield and said, “I like this shield.”

(There's actually a third reason, but I'm sure I have bored you already.)

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You have explained this idiom extremely well.  I truly hope you will write further notebook entries, explaining further Chinese idioms.