In folklore, a bullet cast from silver is often one of the few weapons that are effective against a werewolf or witch. The term  is also a metaphor for a simple, seemingly magical, solution to a difficult problem: for example, penicillin was a silver bullet that allowed doctors to treat and successfully cure many bacterial infections.
In folklore, a bullet cast from silver is often one of the few weapons that are effective against a werewolf or witch. The term silver bullet is also a metaphor for a simple, seemingly magical, solution to a difficult problem: for example, penicillin circa 1930 was a "silver bullet" that allowed doctors to treat and successfully cure many bacterial infections.
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silver bullet
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was a silver bullet
circa 1930 was a "silver bullet"