Cautionary Tale

Over the summer I moved my glass paper weights onto the window sill as they looked so pretty in the sun. I don't dust or polish as much as I should but I was confused when I moved this Caithness Glass Weight and saw the black mark on the wood underneath. It was like charcoal. I was worried that we had some kind of bonkers dry rot.
When Steve (science teacher) came home he clocked that it was , of course , charcoal indeed and the paperweight had focussed the sunlight resulting in the scorched wood. Scary, could have caused a fire.
He also clocked that it was at least July since sill last polished!