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When did Johnny Cash start performing?
Johnny Cash began performing concerts at prisons in the late 1950s. He played his first famous prison concert on January 1, 1958.
Where was Johnny Cash's first famous prison concert?
Johnny Cash played his first famous prison concert on January 1, 1958, at San Quentin State Prison.

Question: Why did he perform at prisons?
Self-contained rewrite: Why did Johnny Cash perform at prisons?