Lewis Carroll’s 1865 novel “Alice’s Adventures in Wonderland” famously features an eccentric character called the Hatter, who’s referred to in the story as “mad” and became popularly known as the Mad Hatter. However, the phrase “mad as a hatter,” used to describe someone who’s crazy or prone to unpredictable behavior, didn’t originate with Carroll. Instead, the expression is linked to the hat-making industry and mercury poisoning. In the 18th and 19th centuries, industrial workers used a toxic substance, mercury nitrate, as part of the process of turning the fur of small animals, such as rabbits, into felt for hats. Workplace safety standards often were lax and prolonged exposure to mercury caused employees to develop a variety of physical and mental ailments, including tremors (dubbed “hatter’s shakes”), speech problems, emotional instability and hallucinations.