Why do we say 'mad as a hatter'?

Hat-making used to be surprisingly dangerous work, as the manufacture of felt involved mercury. Working in confined spaces, hatters couldn’t avoid coming into contact with the substance or breathing in harmful vapours.

The mercury damaged their nervous systems, causing them to shake involuntarily, and affect their mood. One account said that a hatter was “easily upset and embarrassed, loses all joy in life... and may lose self-control”.

The afflicted would lose coordination and memory, while growing nervous, irritable and dizzy. It wasn’t until the 20th century that the risks of mercury poisoning (still known as 'Mad Hatter’s Disease' today) forced changes in the hat-making industry.

The real mad hatters, however, weren’t the inspiration for Carroll. Instead, it may have been an eccentric furniture dealer and inventor always seen in a top hat, Theophilus Carter. Among his creations was an ‘alarm clock bed’ – think Wallace and Gromit – which woke sleepers by tipping over.