There are countless ways to use everyday items to make a fire in survival situations. From short-circuiting cell phone batteries to refracting light off the bottom of a soda can, makeshift techniques using what you can scrounge up from your environment can save your life. Creating fire with a water bottle is similar to making a fire with a magnifying glass: the water bottle acts as a lens that focuses sunlight into a super-bright point which quickly heats up paper or other tinder to the point of combustion.
The first thing to know about making fire with a water bottle is that it’s not easy. It takes time and practice to get the bottle’s position just right, and if the sun isn’t at its full strength because it’s late in the day or behind clouds, your efforts might go unrewarded. To maximize your chances of success, use clear bottles with smooth, curved tops that are filled with clear liquid. Colors absorb light and textures hinder your ability to focus the light. For tinder, paper with dark ink works best — the dark ink absorbs heat and ignites faster.
(via The Art of Manliness)