While car headlights and flashlight reflectors are ready-made parabolic reflectors, they aren’t the only ones around. The advantage those two have is that they are already polished, so they don’t require any extra work, other than taking them apart. However, there are several other things which can be used as parabolic reflectors, although they do require more work.
Soda cans are commonly found trash items that you can find nearly anywhere, provided that you’re not in some ultra-rare, uncharted area that man hasn’t already been. The bottom of most soda cans is a fairly good parabolic dish, although it typically has a matte finish, which will not focus the light and start your tinder burning. That can be easily remedied though. All it needs is polish it to make it shiny. While you probably don’t carry a jeweler’s buffing wheel around in your pocket, there’s a good chance that you will have something that can be used to polish it.
Toothpaste actually makes an excellent polishing compound. You can use the toothpaste either on a piece of cloth, such as a bandana, or directly with your fingers. Another option, which many people always try to have with them, is chocolate. Yes, believe it or not, that chocolate bar you have been saving for a rainy day is actually perfect for polishing. The cocoa powder in it is very hard, albeit finely ground. The cocoa butter in the bar acts as a lubricant, working together with the cocoa powder. One final option for polishing the bottom of the soda can is the ash from an old fire.
Don’t expect your polishing efforts to be over in a matter of minutes; you’re probably going to have to work at it for a half-hour or more. Nevertheless, when you get done, you’ll have a nicely polished reflector, which can be used to focus the sun’s light onto your tinder to start a fire.
This is another great party trick, assuming you bring your own can that’s already polished. Your friends won’t be able to figure out how you got it to work, because they won’t be able to do it with their cans. Just make sure you hide your can after starting the fire.