Seven Bridges of Königsberg
First, Euler pointed out that the choice of route inside each land mass does not matter. The only important property of a route is the order in which the bridges are crossed. So, he changed the problem to abstract terms. This laid the foundations of graph theory. He removed all features except the list of land masses and the bridges connecting them. In the language of graph theory, he replaced each land mass with an abstract "vertex" or node. Then he replaced each bridge with an abstract connection, an "edge". An edge (road) recorded which two vertices (land masses) were connected. In this way, he formed a graph.