In a map coloring, adjacent regions have different colors. According to the four-color theorem, only four colors are needed to color any map. The map shown here was introduced for the first time by Martin Gardner in 1975 [1]. You can color it by clicking the polygons; clicking the same polygon repeatedly cycles through the colors.

[1] M. Gardner, "Mathematical Games: Six Sensational Discoveries That Somehow or Another Have Escaped Public Attention," Scientific American, 232(4), pp. 127–131, 1975.