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.

Talking about plastic bottles...

Some facts about plastic bottles

As recycling rates remain dismally low, making bottles requires virgin materials, namely petroleum feedstocks. It takes 162g of oil and seven litres of water (including power plant cooling water) just to manufacture a one-litre bottle, creating over 100g of greenhouse gas emissions (10 balloons full of CO2) per empty bottle. Extrapolate this for the developed world (2.4m tonnes of plastic are used to bottle water each year) and it represents serious oil use for what is essentially a single-use object. To make the 29bn plastic bottles used annually in the US, the world's biggest consumer of bottled water, requires more than 17m barrels of oil a year, enough to fuel more than a million cars for a year.

Makes you think doesnt it?