There are few cartoon tropes as ubiquitous as killing ants with a magnifying glass. But if you don't have a glass, you can always just use the next best thing: A Ziploc bag and some water.
Image: YouTube Screenshot
Grant Thompson, who runs the King of Random YouTube channel that features a lot of DIY projects and LifeHacks, demonstrates the method for Veritasium: He grinds up some dried bark as a fuel, then fills a Ziploc bag with water and twists it until it becomes a sphere. See-through spheres act like biconvex lens, where the two opposite roundest sides of the bag become the two lenses.
These double-lenses heat things because light slows down when it passes through them, and then bends with the shape, bending more in the thinner section and less in the thicker section. This concentrates the sunlight into a single focal point, all of it adding heat onto the combustible fuel source, in this case the dried up bark. Then it's just a matter of feeding the fire with more fuel and more oxygen.
I don't doubt this works — folks have been starting fires with focused sunlight since the ancient Greeks. There's a myth that Archimedes even used a big mirror to set fire to invading Roman ships.
The fact that one can find enough dry wood to grind into a fine powder baffles me. But then again, it's rained on almost every camping trip I've ever been on.