Place a fairly large sheet of plastic into a bowl. Fill it full of water, gather up the ends and begin to twist all the edges together making a ball.
On a clear sunny day hold it over something combustable, raise and lower it until the light passing through the water is focused onto a very small point. In seconds the spot will begin to smoke, a few puffs of breath will ignite the material. Then all you need to do is add fuel.
The same principle can be used with a water bottle or any container that holds water as long as it is clear. This idea is derived from using a magnifying glass.
I have as well used my eyeglasses to do the same… have fun.