Generally, we attribute the birth of graph theory to the famous problem of the Königsberg bridges that captivated the Prussian bourgeoisie in the 18th century. The City of Königsberg, on the Pregel River, had 7 bridges and the question was to find out if you could imagine a route through the city that took each of the 7 bridges only once to return to your point of departure. We now know that such a route is not possible, since Euler demonstrated the impossibility of this route. To do that, Euler defined a kind of traversable graph that are now known as Eulerian graphs. These are graphs that pass through all edges and that you can draw without lifting your pencil.