A clear bottle and water is all you need to make fire.

Because you can find water and clear bottles, virtually everywhere, you have all you need to get your fire started.

This takes practice as you have to get the angle right to maximize the effect of concentrated sunlight and get the hottest burn point possible, so you should try this before you find yourself needing to do it to make fire.

That said, however, once you have done it a few times, it becomes easy to repeat it when you need it.