a melting mystery
when I was a teenager, we came home from a shopping trip and followed the routine of "put the kettle on and make a cup of tea while putting away".
I picked the kettle up and noticed that there was a curved melt mark in a shallow crescent with a hole at the centre of the curve.
I showed this to my mother and father and we puzzled for more than a few minutes (after locating the top-of-stove whistling kettle to make tea) what couold have caused it.
I worked it out when I noticed that my father had been shaving in the kitchen - the convex side was facing the rear (south facing) kitchen window and the kettle body was at the focal length.
As the sun tracked the sky, the hotspot at the focus basically melted and burned through the plastic kettle.