'I saw this method in a YouTube video a while back ago and thought it was pretty damn cool. All you need is a soda can, a bar of chocolate, and a sunny day.
Polish the bottom of the soda can with the chocolate. Open up your bar of chocolate and start rubbing it on the bottom of the soda can. The chocolate acts as a polish and will make the bottom of the can shine like a mirror. If you don’t have chocolate with you, toothpaste also works.
Make your fire. After polishing the bottom of your can, what you have is essentially a parabolic mirror. Sunlight will reflect off the bottom of the can, forming a single focal point. It’s kind of like how a mirror telescope works.
Point the bottom of the can towards the sun. You’ll have created a highly focused ray of light aimed directly at your tender. Place the tender about an inch from the reflecting light’s focal point. In a few seconds you should have a flame.
While I can’t think of any time that I would be in the middle of nowhere with a can of Coke and chocolate bar, this method is still pretty cool.'