The term "mad as a hatter" will forever be linked to the madcap milliner in Lewis Carroll's classic children's book, Alice in Wonderland. But few actually know that the true origin of the saying relates to a disease peculiar to the hat making industry in the 1800s. A mercury solution was commonly used during the process of turning fur into felt, which caused the hatters to breathe in the fumes of this highly toxic metal, a situation exacerbated by the poor ventilation in most of the workshops. This led in turn to an accumulation of mercury in the workers' bodies, resulting in symptoms such as trembling (known as "hatters' shakes"), loss of coordination, slurred speech, loosening of teeth, memory loss, depression, irritability and anxiety -- "The Mad Hatter Syndrome." The phrase is still used today to describe the effects of mercury poisoning, albeit from other sources.