It's the big paradox of emergency-use flashlights ... by the time you eventually need to use them, their batteries have died. Eton's new Blackout Buddy H2O, however, will reportedly still work after sitting for up to 10 years. And to turn it on, you just add water.
This latest member of the Blackout Buddy line has a magnesium-oxide battery, which starts delivering power to the light's three LEDs when exposed to H2O. To initially fire it up, you dip it into a small cup of water, or pour water into its battery compartment. After that, it will keep going continuously for up to 72 hours – if it starts to dim within that time, you simply add more water.
The device is only about the size of a cigarette lighter (so it can easily be stashed), and is claimed to even work underwater.
It is a one-time-use product, so it's just as well that its battery contains no toxic mercury, hexavalent chromium, cadmium or lead. It also only costs US$10, so you won't be throwing away an expensive item once you're done with it.
Source: Eton via Popular Science
Urine will work. I didn't ask about saliva.
Eton Corp's tech support manager says it will work with straight urine.
I'm going to grab a few.