Calcium Carbide (Rock+Water=Fire?)

Calcium carbide is something you don't see much anymore, but before electric light was ubiquitous and cheap, it was some amazingly useful stuff. Calcium carbide is a boring looking compound - I've only ever seen greyish lumps. It eacts like a salt of doubly deprotonated acetylene (C₂H₂, or here, more like C₂^2-). Anything that reacts like (I say "reacts like" because it's surely a little more complicated than the structure I give below, but it's close enough for us) negatively charged carbon will be pretty basic, and tend to grab a proton wherever it can. One ubiquitous proton-bearing chemical is water. The reaction of CaC₂ with water gives Ca(OH)₂ and acetylene - a flammable gas:

Quite a bit of energy is stored in that acetylene molecule, and it will burn pretty nicely. This will make a handy lamp/handwarmer, especially if you're a turn-of-the-20th-century bicyclist, caver, or miner.

The nice thing about calcium carbide is that it is a source of a flammable gas that is easily controlled (by the rate of water you drip on it) and easy to handle. Gases are a pain to handle - we can compress them in cylinders, but only to a point. This is one big reason hydrogen is such a tricky energy storage medium - it's very much a gas, and only liquifies at temperatures within throwing distance of absolute zero.

Apparently, calcium carbide was preferred by some spelunkers for lighting purposes over batteries until quite recently, since the amount of light you get per unit weight is pretty good. It is a very unique compound, not just in the fact that it can evolve gas with water - lots of compounds can do that (see, for example, the butyllithiums, some of which will give a beautiful violet lithium flame in the presence of humidity - or even oxygen). The beauty of calcium carbide is that it does this in a controlled, non-blowing-up way.