Wikipedia
Thiotimoline
Thiotimoline is a fictitious chemical compound conceived by science fiction author Isaac Asimov. It was first described in a spoof scientific paper titled "The Endochronic Properties of Resublimated Thiotimoline" in 1948. The major peculiarity of the chemical is its "endochronicity": when it is mixed with water, it starts dissolving before it makes contact with water.
Asimov went on to write three additional short stories, each describing different properties or uses of thiotimoline.