Hello, I'm an expert in numismatics, the study or collection of coins. I'd be happy to help you with your question about Kennedy half dollars.

English Answer:
Kennedy half dollars are not pure silver. They are composed of a copper-nickel clad composition. The composition of the Kennedy half dollar has changed over the years. From 1964 to 1970, these coins contained 40% silver, but since 1971, they have been made of a core of copper-nickel (75% copper and 25% nickel) and a layer of copper over the surface. The reason for the change was the rising price of silver and the need to reduce costs in minting coins.

Chinese Translation:
您好，我是硬币研究或收藏领域的专家。我很高兴能帮助您解答有关肯尼迪半美元硬币的问题。

英文回答：
肯尼迪半美元硬币不是纯银的。它们由铜镍包覆组成。肯尼迪半美元的成分随着时间发生了变化。从1964年到1970年，这些硬币含有40%的银，但自1971年以来，它们由铜镍（75%的铜和25%的镍）的核心和表面覆盖的铜层制成。变化的原因是银价上涨以及减少铸币成本的需要。

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The Kennedy half dollar, first minted in 1964, is a fifty-cent coin currently issued by the United States Mint. ... Starting with 1965-dated pieces, the percentage of fine silver was reduced from 90% to 40% (silver clad), but even with this change the coin saw little circulation.read more >>