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Have you ever heard the term "mad as a hatter"? Long ago, hatters, or hat-makers, went crazy (mad) because of the mercury they used to cure fur pelts. Today, we know that mercury poisoning causes tremors, problems with vision, hearing, speech, and even hallucinations. These were the marks of Mad Hatter disease.
Why did hatters go mad?
What is "because of the mercury they used to cure fur pelts"