You can start a fire by striking the blade of your closed pocketknife against the sharp edge of a rock. The rock must be hard enough to shave tiny slivers of molten steel from the knife. Look for flint, chert, quartzite, jasper, obsidian, or granite. Carry a knife with a carbon-steel rather than a harder stainless-steel blade. CATCHING SPARKS: Press a bit of tinder fungus or char cloth between thumb and rock, then flick the back of your blade against the chip's sharp edge so that sparks land on the tinder. When the material is glowing, transfer it to a bundle of bark shavings or dry grass and gently blow to flame. Look for tinder fungus, which has a charred appearance, growing on the bark of living birches. Make char cloth by burning bits of old jeans, tamping the blackened pieces into a jar as the flame dies down, then tightly shutting the lid. —KEITH McCAFFERTY