A fictional chemical
invented by Isaac Asimov
Thiotimoline supposedly dissolves in water around one second before it touches the water itself. This property appears to introduce a time travel paradox: what if you decide to pour water onto thiotimoline, but change your mind when you see the thiotimoline dissolve? Asimov determined that once the substance dissolved, it would become wet somehow. Perhaps you would be startled and accidentally pour the water on it after all.
The practical applications of this remarkable substance become apparent when you imagine a device which will add water to the thiotimoline in another cell if and only if the thiotimoline in its own cell dissolves. Hook up ten of these in series and the final cell will dissolve just over ten seconds before you activate the first cell in the battery. Since the thiotimoline cannot be fooled, the end of the battery will somehow become wet.
From this invention, Asimov extrapolates to create a doomsday weapon: a thiotimoline bomb consisting simply of a fuse of a given duration -- 72 hours or so. You have to wait for the fuse to begin reacting, but once the process has begun, you just seal the fuse in an absolutely waterproof container and put it near your target, preferably in a place (like the top of a building) where water would not normally be. It is guaranteed that through some event -- a flood, for example -- the fuse will be activated 72 hours after it has begun dissolving.
Thiotimoline made its first appearance in The Endochronic Properties of Resublimated Thiotimoline, a spoof of chemical journals and scientific writing in general. It has since appeared in other short fiction by Isaac Asimov and (as homage) the works of other sf authors.