An Eulerian path is one that hits every edge of a graph no more than once. The Eulerian trail puzzle is one that takes this idea and turns it into a problem that demands solving.
Upon first looking at the Eulerian puzzle on this video, the solution may seem too easy to take seriously. There’s a rectangle with five inside of it (including two squares). All you have to do to complete the puzzle is draw a path through every line in the diagram and then bring it back to form a full loop. The problem is that you can’t pass through the same line twice. There are 16 total lines to cross and, again, things looks fairly easy, that is until you only have about one or two left. It’s then you find out that there’s actually no way to do this. While this puzzle looks like a childlike challenge, it’s actually impossible to complete.