The city of Königsberg was located in Prussia on the Pregel River. Two islands in the river were connected by seven bridges. By the eighteenth century, it had become a tradition for the citizens of Königsberg to go for a walk through the town trying to cross each bridge only once. No one was able to succeed, and the question was asked whether it was possible to do so. In 1736, Leonhard Euler (1707–1783) proved that it was not possible to cross the Königsberg bridges only once. Euler’s solution led to the development of two new areas of mathematics: graph theory, which deals with questions about networks of points that are connected by lines; and topology, which is the study of those aspects of the shape of an object that do not depend on length measurements.