In the early 18th century, the city of Königsberg (now Kaliningrad, Russia) was a bustling Prussian port built around the Pregel River. The river split the city into four distinct land areas — two riverbanks and two islands — connected by a total of seven bridges . The citizens of Königsberg enjoyed strolling through their city, and a particular challenge became a favourite topic of conversation: Is it possible to take a continuous walk that crosses each of the seven bridges exactly once? You could start anywhere, end anywhere, and walk in any order — but you could not cross the same bridge twice. Map of Königsberg in Euler's time showing the actual layout of the seven bridges, highlighting the river Pregel and the bridges (Image: Wikipedia) The layout Imagine standing on one of the riverbanks. From where you are, you can see bridges leading to both islands, and from each island, more bridges leading to the other island and to the opposite bank. The arrangement is symmetric enoug...