Water bottle fire starting is all about creating a lens (like a magnifying glass) and concentrating the heat from sunlight onto tinder. For the best chance of success, the water in the bottle should be crystal clear, and the round shoulder of the bottle should be clear, smooth and un-textured. This is supposedly why many bottle manufacturers pay a little extra for a textured bottle, so their product doesn’t get bad press from being the cause of destructive wildfires. You’ll also need some top-quality tinder. Char cloth is ideal, due to its low ignition temperature and dark, light-absorbing color. But for my experiment, I used birch fungus powder.