This cover from the July 1965 issue of Scientific American illustrates the "Four Bugs Problem" featured in Martin Gardner's "Mathematical Games" column about op art. The setup: Four bugs are placed at the corners of a square. They start crawling clockwise (or counterclockwise) at a constant rate, with each bug moving directly toward its neighbor.
As the bugs move, they always form the corners of a square...
Mark Frauenfelder.
