How fires start?

We hold our winter pool cover down all round the edge with filled 8 litre water bottles, which has worked very efficiently but as I was tidying up around the pool yesterday, and I noticed some black holes and marks on the pool winter cover. Further investigation revealed it was burns, what has happened is the sun has been magnified through the bottles of water to cause intense heat spots and has nearly set the cover on fire, if someone had told me this was possible I would not have believed them, I now wonder if this is how some of the fires start in the campo.

The cover will be coming off in a couple of weeks for the solar cover to go on, but in the meantime have painted the bottles with white emulsion to stop the suns rays causing any further damage.