Fluidity.

[Markiwi]

What is fluid at room temperature and drives people crazy

[sigma]

Mercury

[stewey]

Alcohol?

[eddiebun]

Great answer Stewey !

As to sigma's answer that's where the phrase as mad as a hatter comes from.

From the mid-18th to the mid-19th centuries, a process called "carroting" was used in the making of felt hats. Animal skins were rinsed in an orange solution of the mercury compound mercuric nitrate, Hg(NO3)2�2H2O. This process separated the fur from the pelt and matted it together. This solution and the vapors it produced were highly toxic. Its use resulted in widespread cases of mercury poisoning among hatters. Symptoms included tremors, emotional lability, insomnia, dementia and hallucinations. The United States Public Health Service banned the use of mercury in the felt industry in December 1941. The psychological symptoms associated with mercury poisoning may have inspired the phrase "mad as a hatter";