This picture is part of an example of what is known as a trapdoor function .
Trapdoor functions are important in cryptography.
Elsie the Entrepreneur wants to set up espresso stands on the street corners and get rich off of the populace's all-consuming desire for flavored lattes with sprinkles. Elsie's highly scientific market survery has revealed that people are willing to walk a block for their trendy caffienated drinks and Italian sodas, but if they had to walk further than that, they would just get their drinks from the vending machines in the basements of their buildings. It isn't terribly hard to solve the puzzle using the tiny map at the top of the page. How about a big copy of harder one so you can print it off and use scraps of paper or plastic chips to indicate the location of the espresso stands, see if you can solve the problem. In fact, there is no known algorithm for solving this problem quickly and accurately for an arbitrary map of a town.