How Does the Universe Work? A Puzzle Analogy
The shapes are squares and triangles. The connecting rules are that triangles must connect on one and only one side, while squares must connect on two and only two sides.
Puzzle adapted from Helen Quinn, "Of Quarks, Antiquarks, and Glue," The Stanford Magazine, Fall, 1983, p. 29. and included in the Topics in Modern Physics, May 1990, and Catching the Sun, 1992 Fermilab.