Fire with magnifier

I suppose you have heard of a fire stone, almost everyone would have. These are the stone that were used in olden days to light flammable substances like sticks. Did you know that even magnifiers could be used to start fire? The operation is something like this: you place the flammable substance such as cotton on the ground and then hold the magnifying glass so that the sun rays is focused on the cotton. This produces heat and subsequently the cotton catches fire. Not, only a magnifying glass, even the bottom of a bottle could be used. This is the reason that littering the forest is prohibited in some countries, fearing that if the bottom of the bottle is positioned in an optimum manner so as to make the underlying dry leaves to catch fire, it would lead to a great forest fire. So, be careful while dealing with glasses, you not only have the danger of cuts, but also that of a fire.