In the first half of the 18th century citizens of Königsberg were very proud of their city and the seven bridges located in it. It is said that city had great views near the river, so the citizens liked to take walks by the river. While on a walk they, obviously, had to traverse the bridges. Naturally the question emerged - would it be possible to choose path so that it would allow to traverse each bridge only once. The answer and the analysis of the problem maybe found in this video by Numberphile.