When people draw maps they often show every part in a different colour from the ones around to stand out clearly. This one shows the old Welsh counties. The coloured shapes fit together like a mosaic.
Map makers discovered that they never needed more than four colours to make sure areas of the same colour never touched. (For this map the makers chose to use more.) Mathematicians were interested why only four colours were needed, however they drew the shapes. It took over a hundred years and computers to find the mathematical answer.