Hopefully you’re counting on more than your ability to start a fire to keep you alive. That means that you keep some basic survival supplies around, such as a bottle of water. Even if you don’t have a plastic water bottle on hand… they are quickly becoming one of the most common pieces of trash on the planet. While it makes a great container for your drinking water, it also has a very valuable property that you may have never thought of. You can use that water bottle as an improvised lens, if you don’t have a better lens available to use for starting a fire.

This works best if you have a clear plastic water bottle, rather than a colored one. Colored plastics will block out some of the light, reducing the effectiveness.

Another important factor is how smooth the water bottle is. A Nalgene water bottle will have smooth sides, whereas the water bottles you buy in the grocery store or convenience store won’t. The indentations on the side of the water bottle, which are there to keep the thin plastic from collapsing, will refract some of the light into other directions that what you want, reducing your lens’ effectiveness.

To use a water bottle, be sure that it is clean and remove all labeling. Fill it all the way with clean water. You don’t want any air bubbles in it either. While this water needs to be clean in the sense that it doesn’t have any dissolved solids in it or things floating around in it, it doesn’t need to be drinkable. Lay the filled water bottle on its side and hold it over your tinder as a lens in order to start the fire.