I have an adjustable screen for my window, the kind you expand horizontally to fill up the windowsill. It’s somewhat effective at keeping bugs out of the house.
When it’s not opened all the way up, the two layers of screen overlap in the middle. Depending on the angle you are looking from, you can see some cool images.
At this angle, for example, I see a contour map of a function of several variables.
I wish I understood where the curves come from!