A man of the state of Chu had a spear and a shield for sale. He shouted loudly to praise his shield.
　　 “My shield is so strong that nothing can pierce through it.”
　　 He also sang praises of his spear.
　　 “My spear is so sharp that it can pierce through anything.”
　　 “What would happen,” someone asked, “if your spear is used to pierce your shield?”
　　 He was unable to give an answer.
　　 It is impossible for an impenetrable shield to coexist with a spear that finds nothing impenetrable.
　　
　　① shield n. 盾牌
　　② pierce v. 刺
　　③ impenetrable adj. 不可穿透的
　　
　　 有一个楚国人在叫卖矛和盾。他自夸说：“我的盾非常坚固，没有任何矛能穿透它。”又夸他的矛说：“我的矛锋利无比，任何坚固的东西都能穿透。”有人反驳道：“用你的矛戳你的盾会怎么样？”他无言以对。坚不可摧的盾和无坚不摧的矛不可能同时存在。
　　
　　阅读短文，回答下列问题：
　　1. What did the man of the state of Chu have?
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　　2. Why was he unable to give an answer?
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　　3. What can we learn from this simple story?
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