… to little green puills, and get Martians.
… to Gremlins, and get more Gremlins
… to lithium, sodium, potassium, or rubidium and get hydrogen and heat, resulting in a floating fire
… to calcium carbide, and get acetylene
—to calcium oxide, and get heat and a smoke screen
— to lithium hydride, and get hydrogen gas and NO heat, so you can fill your balloon
… to calcium chloride, and get warm
what else, fictional or otherwise, can you add?
… to sodium bicarbonate, and get cool.
In the Joel Rosenberg’s Sword and the Chain series, you added a little water to a little pile of powerdery residue to create an explosion, mimicking the effects of gunpowder.
I remember a short story where an alien had a glass gun that mixed water and acid to fire flechettes.
Add water to iron oxide powder to heat your MREs.
Stanley G. Weinbaum’s “A Martian Odyssey” (and its sequel). A classic from the 1930s.
Just add one small drop of water to a mix of aluminium and iodine powder to get flame and purple smoke, as I fondly remember from chemistry class 35 years ago.
Just add water to a white powder (anhydrous copper sulphate) to get clear blue liquid.
Just add water to a pan full of boiling vegetable oil to get a fireball (thanks to our local fire station for the safety demo at the village fete).
Just add water to the planet Mars to visit genocide on the few remaining natives (Protector, Larry Niven).
to spoiled powdered milk to get a good intoxicating drink for Tenctonese
Just add water …
to Evil Witches in Oz to get a melted evil witch.
to a heated Deep Fat Frier to get a nasty exlosion of skin burning hot oil and water. 
… to air to get “Instant Water”.
Just add water…
To those cool multi-colored crystals to get a forest of dendrites.
I loved those as a kid!
…to carbon and make sugar!
…to an acorn and get an oak tree.
(Yeah, I know it’s not quite that simple)
…to the grubby pile of dirt ambulating around the living room, and get a toddler!
If you’re Wile E. Coyote just add water to a small pill and get an instant tornado!
Then accidentally drop water into the whole jar of pills(they’re Acme of course) and get a really huge tornado that sweeps you away!
…to those Aliens in Signs, and…nevermind.
…from a water cooler in a bank next to a burning building and get your powers back. (Frozone only)
…to the floor of a computer room in the Pentagon and have the security system sound the alarm and lock down the room. (Warning: security system designed to prevent file copying to disk does not actually detect the decibel level of a disk copying files)
…spread the seeds, and watch it grow! CH-CH-CH-CHIA PET!!
I have a package of instant water… I don’t know what to add…
[/Steven Wright]
S^G
to Bourbon, and get drunk.
Add A LOT of water and get a flood.
…to flour and get glue.
… to a puddle and get a bigger puddle
(x+x=2x, except for number of puddles)
. . . to Mr Fusion and you can run your time machine.
There’s a microscopic critter, don’t recall the name, which can lose 99+% of it’s water, and go into a sort of suspended animation. It can then blow around like dust, until perhaps years later it lands somewhere damp and it absorbs the water and reanimates.